Since Britain voted to leave the European Union more than a year ago, Europeans have mockingly said that the decision will result in an isolated, lonely island nation.

But Britain, in fact, already has a serious problem with loneliness, research has found. More than nine million people in the country often or always feel lonely, according to a 2017 report published by the Jo Cox Commission on Loneliness.

The issue prompted Prime Minister Theresa May on Wednesday to appoint a minister for loneliness.

“For far too many people, loneliness is the sad reality of modern life,” Ms. May said in a statement.

“I want to confront this challenge for our society and for all of us to take action to address the loneliness endured by the elderly, by carers, by those who have lost loved ones — people who have no one to talk to or share their thoughts and experiences with.”

Mark Robinson, the chief officer of Age UK, Britain’s largest charity working with older people, warned that the problem could kill.

 “It’s proven to be worse for health than smoking 15 cigarettes a day, but it can be overcome and needn’t be a factor in older people’s lives,” he said.

A former United States surgeon general, Dr. Vivek Murthy, wrote an article for the Harvard Business Review last year arguing that loneliness needed addressing in the workplace.

It could be associated, he wrote, “with a greater risk of cardiovascular disease, dementia, depression and anxiety.”

The British report was commissioned by the Red Cross in partnership with the Co-op, a cooperative supermarket chain, and published by the Cox commission in December.

The group operates in memory of Ms. Cox, 41, a Labour Party lawmaker who was shot dead by a right-wing extremist in 2016, and who had been a prominent voice in Parliament on the issue, setting up a cross-party commission that aimed to start a national conversation and establish the scale and impact of loneliness in Britain.

The prime minister announced on Wednesday that Tracey Crouch, who is the under secretary for sport and civil society in the culture ministry, would lead a governmentwide group to build on Ms. Cox’s legacy and establish policies on the issue.

In parallel, the Office for National Statistics would help to establish a method of measuring loneliness, and a fund would be set up to help the government and charities to develop a wider strategy to identify opportunities to tackle the problem.

The Cox commission, chaired by the lawmakers Rachel Reeves and Seema Kennedy, said it welcomed the government’s “prompt response” to its report.Quoting Ms. Cox, the lawmakers said in a joint statement, “Young or old, loneliness doesn’t discriminate.”

“Throughout 2017 we have heard from new parents, children, disabled people, carers, refugees and older people about their experience of loneliness,” they added.

Government research has found that about 200,000 older people in Britain had not had a conversation with a friend or relative in more than a month.

Carol Jenkins, 64, a retired nurse from Berkshire, in southwest England, said she started to feel lonely when her son moved abroad and she downsized to a smaller house in a different county.

“It was a financial decision to move, and I didn’t really have it in me to start making new friends,” Ms. Jenkins recalled on Wednesday in a phone interview. “Months would go by without seeing my friends or family, and I felt really depressed and alone.”

Ms. Jenkins has since joined a Facebook group for Britons affected by loneliness, which, she says, has helped her to get out of the house more.

“It’s not so much about meeting people on the internet and making new friends, but it’s more of a motivational support network that gives you direction on how to cope and fix the problem,” she said, adding that she was surprised by how many young people had joined the group.

“There are so many university students who just lock themselves in their rooms for days because they feel rejected or that they don’t fit in,” Ms. Jenkins said. “It’s only a matter of time before loneliness turns into depression. And that’s where it gets dangerous.”