Labour leader Jeremy Corbyn. (File photo: VCG)

Britain's main opposition Labour party on Tuesday sought to pile pressure on the ruling Conservatives over state-run healthcare in a bid to exploit a government weakness and divert attention from its message on Brexit.

Opinion polls show Johnson's Conservatives enjoying a comfortable lead before an election that Prime Minister Boris Johnson has called the "most important in a generation".

Johnson's minority government hopes to secure a parliamentary majority on Thursday to help him pull Britain out of the European Union by the end of next month.

But a ruling party memo published by the pro-Conservative Daily Telegraph newspaper warned that just 40,000 votes in 12 constituencies could see Labour leader Jeremy Corbyn becoming prime minister.

The veteran socialist is trying to stage a late surge by focusing on Tory funding of the taxpayer-funded National Health Service (NHS) since the party came to power nine years ago.

Labour claims nearly 4,700 deaths recorded between October 2018 and November 2019 could be attributed to "patient safety incidents" that resulted from NHS staffing constraints.

"The NHS has to be properly funded and at the moment it isn't," said Corbyn, who has warned that the Tories want to sell off the NHS to private firms, threatening its key principle of free treatment for all.

"All research shows there's a very large number of hospitals where patients are at risk because of staff shortages, because of a lack of equipment, because of poor maintenance of hospital buildings," he told BBC television.

"It is a serious issue."

- Tactical win -

Studies suggest that voters place the fate of the NHS as the second-most important issue behind Brexit and Britain's relations with the rest of the world.

They also point to a general mistrust of Corbyn's non-committal position on Brexit -- and frustrations over the Conservatives' handling of the NHS.

Johnson has almost exclusively focused on his government's determination to "get Brexit done" and honour the result of the 2016 referendum on European Union membership.

The Politico news site said "any day that the conversation is about the pressure on the NHS is a tactical win for Labour", with swing voters seen as key to victory on Thursday.

"For the campaign to have been dominated so far this week by a row over (accident and emergency) departments is fairly disastrous for the Conservatives," it added.

- 'Heartbreaking image' - 

Britain's partisan media has assumed a dominant role in the final stretch of a manic five-week campaign that has seen Corbyn and Johnson spend most of their time on the road.

The Labour-supporting Daily Mirror newspaper devoted its first seven pages on Tuesday to the health of the NHS, a day after publishing a front-page photo of a small boy with suspected pneumonia who was forced to sleep on a hospital floor.

It followed up with another of a sick baby resting in an armchair because of a lack of beds.

"Another day, anther heartbreaking image of a sick child that shames the (Conservatives) of starving the NHS of cash," the newspaper wrote.

Johnson drew criticism on Monday when he was confronted on camera and asked to comment on the photograph of the boy on a journalist's phone. 

He refused and instead grabbed the reporter's phone and put it in his pocket. He eventually looked and said it was a "terrible, terrible photo" before giving back the phone.