British Prime Minister Rishi Sunak said on Friday the government would look to tighten rules for long-term sick leave in a bid to reverse a rise in the number of Britons who have permanently dropped out of the workforce.

Labour force participation among working-age Britons is its lowest since 2015, mainly due to a rise in long-term illness and a greater number of students, in contrast to other large, rich nations which have seen increased participation since 2020.

With his eyes firmly on a national election later this year, which polls show he is expected to lose, Sunak sought to appeal to core Conservative voters by warning the current welfare bill was fiscally unsustainable, and arguing that a 'sicknote culture' around mental health needed to be reined in.

"We need to be more ambitious about helping people back to work and more honest about the risk of over-medicalising the everyday challenges and worries of life," he said in a speech in London.

Sunak said that, if re-elected, he would go further on welfare reform, including by empowering authorities to treat benefit fraud like tax fraud.

Some 9.4 million Britons aged 16 to 64 - 22% of that age group - are neither working nor unemployed, up from 8.55 million just before the pandemic, according to official data. Of those, 2.8 million are long-term sick and 206,000 are temporarily ill.

The opposition Labour Party, which has a double-digit lead in polls, said the Conservatives had failed to deliver either a healthy nation or a healthy economy and its own policies would tackle the root causes by driving down healthcare waiting lists.

Last year Britain's budget watchdog said a quarter of people who were off work due to long-term sickness were waiting for medical treatment, although it added that cutting waiting lists to their 2015 length might only get 25,000 back into work.

Over half of the long-term sick reported suffering from 'depression, bad nerves or anxiety', although many said it was a secondary condition alongside their main health problem.

Sunak said medics were too willing to issue repeat notes approving extended sick leave, rather than advising how a person could get back to work.

Sunak said he wanted healthcare workers rather than family doctors to provide "an objective assessment of someone's ability to work and the tailored support they need to do so".

"We don't just need to change the sick note, we need to change the sick note culture, so the default becomes what work you can do – not what you can't," he said.

(Reporting by David Milliken and Alistair Smout; editing by William James and Christina Fincher)