Abba's Agnetha Fältskog has revealed her first new music in 10 years, a solo song called Where Do We Go From Here?

The star says she was lured back to the studio by producer Jörgen Elofsson, who oversaw her 2013 album, A.

"He played me the demo, and the demo was very good," the Swedish star, 73, told the BBC in an exclusive interview.

"Originally, it was another girl singing and I said, 'I don't know if I can do this like she did'. But I did - and it came out very well, I think."

A summery slice of pop, the song finds Agnetha at a familiar crossroads in a relationship, with lyrics asking: "What if you could be the one I remember when I'm old and grey and looking back at life?"

The track, which premiered on Zoe Ball's BBC Radio 2 breakfast show on Thursday, is the first release (and only new song) from the singer's forthcoming album, A+.

Revisited and remixed

The project is built on the foundations of the dignified, orchestrated ballads of her previous album -only this time, they have been radically overhauled to create sleek, radio-friendly pop anthems.

"I heard one of the old songs on the radio and I started to think, what would happen if you would remix and do another version of the album?" she said.

"Not because you don't like [the original], but because it has been 10 years, and what can you do with it?"

She took the idea to Elofsson, who has also written for Britney Spears, Celine Dion and Kelly Clarkson.

"She mentioned it in a very quick way, but I thought, yes, that's interesting," he recalled.

To test the concept, he stripped the album track Back On Your Radio down to its bare essentials, and rebuilt it around a syncopated dembow beat and dreamy Balearic synths.

"We started to listen and I just felt, oh, this is incredible. How can Jörgen do this with my voice?" said Agnetha. "It's the same singing, so I don't know how he's done it. It's like the song has new clothes."

The discopop vibe persists throughout the reimagined album, which floats along on an echoey, nostalgic haze.

It's an approach that will be familiar to anyone who's heard the recent, dance-inspired overhaul of Sir Elton John's catalogue.

"That Dua Lipa and Elton John song [Cold Heart] is for sure an inspiration," confirmed Elofsson. "I just loved the way it's very powerful and pushing you forward, but the beat is really relaxed."

Nervous about new recording

As work progressed on the remixes, Elofsson remembered Where Do We Go From Here? - a song he'd written with young Swedish artist Kamilla Bayrak - and realised it would be a perfect fit.

"I mustered up some guts and played the song to Agnetha and said, 'Wouldn't you just want to do this one?'" he recalls. "And she was like, 'Wow, what a great song'."

Bright, catchy and melodically complex, the track required the star to reach notes she hadn't hit for years.

"I was a bit tense and a bit nervous because when you get older, your voice changes," she explains, speaking in a rare interview.

"I think my voice has dropped a little bit in tone or in pitch. So I maybe sound a little more... not dark, but lower.

"But I still can express a lot and I like to interpret the songs. And I did it, and I think it came out very well."

Elofsson says: "She doesn't really believe in herself from time to time. But when she does, that's when it comes out - that voice from when she was 15 years old and started singing."

This year marks the 55th anniversary of Agnetha's debut single, Jag var så kär (I Was So In Love).

Written while she worked as a switchboard operator, it topped the Swedish charts, launching a career that made her a star in her home country.

Unusually for the time, she composed the majority of her own material.

"I have written songs for a long time, ever since I was 10," she told Swedish teen magazine Bildjournalen in 1968.

"When I get in the mood, I light two candles - preferably red ones - and sit down by the piano. And then I come up with a melody. Mostly a melancholy one."

But something bigger was on the horizon.

Sad songs

As the 'A' in Abba, Agnetha became one of the world's most famous women, scoring 18 consecutive Top 10 hits in the UK alone, equalling a record set by the Beatles.

Through it all, Agnetha maintained her ear for the melancholy - specialising in ballads of love gone wrong, from The Winner Takes It All and Chiquitita to Hasta Mañana and One Of Us.

"I did some of the happy songs as well - Voulez-Vous and Happy New Year - but it was the sad songs that came out strong," she says now.

The band first secured their place in pop history by winning the Eurovision Song Contest with Waterloo in 1974 - even after the UK gave them the dreaded "nil points".

"That was a mistake, I think," laughs Agnetha. "But you English have been so kind to us in the years after."

By the time of their global breakthrough, she and her bandmate/husband Björn Ulvaeus had already welcomed their first child, Linda.

Their second, Peter, was born three years later - but the couple were back at work within days, walking the red carpet for the premiere of Abba: The Movie.

'We were perfectionists'

Her devotion to the children explains why a songwriter of her pedigree never achieved any writing credits on Abba's albums.

"I didn't have any time," she told the BBC in 2013. "They [Benny and Bjorn] asked me a lot - but when we were at home, I just wanted to be with my children."

That's not to say she was passive in the studio. Agnetha often contributed ideas for arrangements and harmonic structure that, in her words, "brightened up the songs".

"We had a lot of fun in the studio but those were long, long days," she remembers.

"We didn't have the technology there is today, so we had to do everything over and over again, the old-fashioned way, and we didn't let it go until we were satisfied with it.

"We all were perfectionists in different ways. We never cheated. We did everything ourselves - the harmonies and the choirs and everything. Maybe that's why it still sounds so classic."

It's no secret that Agnetha longed to be at home, preferring the regular schedule of studio work to life on the road.

"I can't say it was easy," she says. "And Bjorn is the father, so we both had to leave the children. But eventually we agreed that we cannot be away for too long, so we always went home after maybe two or three weeks and stayed there for a while. That helped us."

Is there a part of her that wishes the 3D avatar technology of Abba Voyage - a concert using digital versions of the band - had existed back in the 70s?

"I don't see it that way because it [touring] had its charm, of course, to see all the people that really loved us - and still love us. But it was difficult leaving home."

The singer attended the opening night of Abba Voyage in London last year, sitting in the spaceship-sized auditorium for the duration of the show, surrounded by adoring fans of all ages.

Watching a digital recreation of her younger self on stage was definitely a "strange experience", she says.

"I still don't know how they did it, really, but the result is enormous.

"Mainly I thought, 'Oh, did I dance that well?' I've never been a master at dancing. We went by feeling - what we expressed was how we felt, there and then."

The process of working on the virtual show led Abba back to the recording studio to make their first studio album for 40 years, also called Voyage, earning the quartet their first-ever Grammy nomination.

The band's 50th anniversary falls next year when, coincidentally, the Eurovision Song Contest returns to Sweden.

Benny and Bjorn have already ruled out performing at the show - but could there be a reunion in any other form?

"You never know anything about Abba - but I don't dare to say anything because I don't know what they are planning," she says. "I'd rather be quiet."

The conversation circles back to the solo album. Although the tracklist has been shuffled since 2013, the closing song remains the same: I Keep Them On The Floor Beside My Bed, a rare Agnetha original, which is steeped in sepia-toned reminiscences of a former flame.

It's a theme she has returned to across her career. Why does the topic of loss and searching for love still intrigue her?

"But I mean, every song you listen to is about love," she protests. "That's the way it is. It wouldn't be the same to sing about something political."

But if her lyrical preoccupations have remained constant since that Bildjournalen interview 55 years ago, Elofsson's production does the unthinkable - filtering her voice through vocoders and Auto-Tune and a myriad of contemporary digital effects.

"We've been very careful with those kinds of things," he says. "We wanted to treat her vocals like they're holy, so when we use those things, it's more for effect.

"I remember the first time I had her in the studio and she started doubling her voice [recording a second vocal track to add depth], I started crying because, you know, this is Abba. I was a child again, listening to the radio in the kitchen."

And one thing is clear - the manipulated vocals would never have appeared without Agnetha's seal of approval.

"I'm very proud," she smiles in agreement.

"I've listened to it so much that I can't remember what the original sounds like."