Labour and the Tories have got into a war of numbers over NHS staff overtime, with ministers accusing the opposition of "scoring another own goal".
According to a new analysis of the official NHS Staff Survey by Labour, published on Friday, staff are working one million hours of unpaid overtime every week.
But the Conservatives hit back, claiming that Labour's four-day week plans will leave health service staff working 2.5 million hours of unpaid overtime each week.
The NHS Staff Survey was completed by about 460,000 of the NHS's 1.5 million strong workforce, with nearly 270,000 saying they were working extra unpaid hours each week.
Labour's analysis found that NHS staff worked more than one million unpaid hours a week, and shadow health secretary Jonathan Ashworth said NHS staff were filling the staffing gaps left by Downing Street policies.
He added: "The NHS is in crisis after nearly a decade of Tory cuts and it's a disgrace that its dedicated staff, who always put their patients first, are having to pick up the pieces to fill the gaps left by this crisis made in Downing Street.
"Boris Johnson's Tories have let the NHS, its staff and our country badly down by intentionally slashing funding for staff training and scrapping the nursing bursary.
"Labour's NHS Rescue Plan will restore the nursing bursary and recruit thousands of doctors and nurses that the NHS clearly needs to end the Tory crisis.
"You can't trust the Tories with our NHS and we can't have another five years of this."
There are more than 100,000 full-time equivalent vacancies in hospital and community NHS services - the equivalent of around one-in-nine posts, according to the Nuffield Trust health think tank.
Labour's sums were based the assumptions that staff who said they worked 0-5 hours unpaid a week had worked an average of 2.5 hours, those who said they worked 6-10 hours a week unpaid had worked an average of 7.5 hours, and staff who said they worked an average of 11+ hours unpaid each week had worked 11 hours, the party said.
The party said that with the survey only completed by a small proportion of the NHS workforce, the total unpaid overtime figure could be even higher.
But the Conservatives accused Labour of "scoring an own goal", claiming that adjusting its analysis to account for its four-day week plans would leave NHS staff doing more than twice as much free overtime.
The party said its calculations were based on Labour's plan to reduce the working week to 32 hours, meaning that overtime would be calculated as extra time worked above 32 hours a week not 37.5.
Health Secretary Matt Hancock said: "Another day, another example of how Labour's four-day week would hit staff.
"Dedicated and hardworking staff would find themselves working up to 2.5 million unpaid hours a week under Labour's plans.
"Our NHS staff can't afford the cost of Corbyn."
The Lib Dems also waded into the debate, with health spokeswoman Luciana Berger saying part of the staff shortages was due to the net loss of 5,000 EU nurses in the last two years.
Ms Berger said Labour's support for Brexit was "baffling" because it would be damaging to the NHS while the Conservatives planned to impose a nurse tax on any new EU health professional coming to treat NHS patients.
She added: "The stakes could not be higher. Labour and the Conservatives must stop being so irresponsible with our NHS.
"The Liberal Democrats will stop Brexit to protect our NHS. We will build a brighter future by investing an extra £35 billion in our NHS by adding a penny on income tax."