Gary Neville has called on the Premier League to stop the transfer of players to Saudi Arabia until it is certain the integrity of its competition is not being put at risk.

Cristiano Ronaldo, Karim Benzema and N'Golo Kante have already joined the Saudi Pro League - marquee names but near the end of their career - and there was speculation Lionel Messi would too, before his move to the US.

A growing number of players at their peak are attracting interest too, including Wolves captain Ruben Neves, Chelsea's Kalidou Koulibaly and Hakim Ziyech and Arsenal's Thomas Partey. It underlines the league's ambition to be one of the top five in the world.

Neville, though, is among those asking what all this potential activity means.

In June, Saudi Arabia's Public Investment Fund (PIF), which owns Newcastle United, confirmed it was taking over four leading clubs in the country, including Al-Nassr, who signed Ronaldo in December.

There is uncertainty over whether PIF holds a stake in Chelsea's ultimate owners, the private equity firm Clearlake Capital, although club sources have rejected suggestions of any direct involvement.

"The Premier League should put an instant embargo on transfers to Saudi Arabia to ensure the integrity of the game isn't being damaged," Neville told BBC Sport.

"Checks should be made on the appropriateness of the transactions.

"If it comes through that process, obviously transfers could open up again. But I do believe, at this moment in time, transfers should be halted until you look into the ownership structure at Chelsea and whether there are beneficial transfer dealings that are improper."

A question of money - and Financial Fair Play

Neville's comments have come as Saudi Arabian spending in football increases, and as some English clubs face a challenge balancing their books.

Both Chelsea and Wolves have been facing difficult decisions around recruitment this summer as they try to keep within the Premier League's three-year limit on losses of £105m under Financial Fair Play rules.

Chelsea spent more than £400m in transfers last season, while Wolves posted a £46.1m loss last year and boss Julen Lopetegui said the club must "solve" their Financial Fair Play issues to compete in the Premier League.

Wolves had thought Neves would join Barcelona this summer. However, the 26-year-old midfielder, who played for Portugal at last year's World Cup, has instead agreed a lucrative deal to join Al Hilal for £47m, a far higher fee than anticipated.

Sources at Chelsea are adamant Saudi Arabia's PIF has "zero interest, financial or otherwise" in the club. It has never been denied PIF have an involvement in Clearlake but it has been stressed the US private equity vehicle has 400 different investors across six continents and it is thought no shareholder is allowed to own more than 5% of the organisation.

Multi-club ownership is not prohibited by European governing body Uefa, which has so far refused to comment. The Premier League does have a fair value assessment system to try to ensure deals, both commercial and transfers, are conducted at market value. Fifa's transfer matching system is designed to do similar.

The aim, ultimately, is to make sure the rules are followed. No easy task, given that markets fluctuate, and a player's value can change.

Will the Saudi Pro League make a big impact on that market?

What's the pedigree of the league?

Saudi Arabia has always had big interest in football - and a competitive league to go with it.

The national team have qualified for six of the past eight World Cups. They have won three Asian Cups - only Japan have more. In Qatar last year, all 26 members of the squad played for domestic clubs. No club have won the AFC Champions League more times than Saudi Arabia's Al Hilal, with four victories.

In recent years, Saudi Arabia has become a more visual presence on the sporting scene, hosting Formula 1 races and high-profile world title boxing bouts as well as setting up LIV Golf.

The controversial purchase of Newcastle was further evidence of a growing interest in using sport to project Saudi Arabia to a wider audience. A joint 2030 World Cup bid with Egypt and Greece is planned too.

"Saudi Arabia sees itself as being at the centre of a new world order and investing in sport helps contribute to that national positioning," Simon Chadwick, professor of sport and geopolitical economy at Skema Business School in Paris, was quoted as saying recently.

Improving the Saudi Arabian league is part of that plan - with the help of high-profile names.

Former Leeds and Wolves forward Helder Costa, former Argentina international Ever Banega and former Watford and Manchester United striker Odion Ighalo all featured last season, when Al-Ittihad, coached by former Wolves and Tottenham boss Nuno Espirito Santo, finished as champions.

Is this another Chinese Super League?

When a league becomes a big player in the world transfer market, questions about its financial sustainability will follow.

The Chinese Super League, for a brief period, attracted much interest and paid huge sums to attract high-profile players, such as former Manchester United and City forward Carlos Tevez. Such levels of spending didn't last.

But the Saudi Pro League believes it is built on more solid foundations.

"The league is well established, going since the 1970s, and clubs have a real fanbase who care about football which makes it authentic and not artificial," a senior league source told BBC Sport.

"When it happened in China, it wasn't explicitly government money. It was about encouraging entrepreneurs to do things. Then that stopped. Here the funding is more secure and part of a long-term plan. The clubs are well established in the local communities and football across the country is by far the number one sport.

"Although the league has a fair amount of foreign players, it's the big stars who get you worldwide TV coverage. As soon as Ronaldo came in, the league started to be shown in every major market. It gets that immediate attention.

"The announcement that the top four clubs will be 75% owned by PIF, rather than by the state, turns them into a proper business. It is not just about bringing in top players but also about changing the economy of the game here to make it more private sector and develop clubs and companies and brands.

"The Ronaldo transfer proved it can happen. It is one thing to say 'we are going to sign the best players in the world' but for someone of Ronaldo's status to actually come, live in Riyadh and play every game, was a surprise to people and showed we could get other people to come."

Is Europe worried about a talent drain?

For European football, the rise of the Saudi Pro League presents a challenge.

Losing key players is not a new experience - China and Major League Soccer have tempted stars away in the past. However, the departure of players in their prime, such as Neves, is a concern.

If he can pick up the kind of eye-watering salary speculated at, how do European clubs respond? Even among the top 20 in Deloitte's rich-list, funds are not limitless and financial regulations have to be adhered to. If they try to compete over an extended period, it would materially impact clubs' biggest outlay - salaries.

And what about the Champions League? If it's pre-eminence as a club competition started to be undermined by an absence of world football's biggest names, could clubs from outside Europe be offered entries?

Uefa president Aleksander Ceferin doesn't see it that way.

"It's mainly a mistake for Saudi Arabian football," he told Dutch broadcaster NOS. "They should invest in academies, bring their own coaches and develop their own players.

"The system of buying players that are almost at the end of their career is not the system that develops football. It was a similar mistake in China when they all brought players who are at the end of their career.

"It's not only about money. Players want to win top competitions. And the top competition is in Europe."