Sir Keir Starmer has admitted that his plan to cut NHS wait times could be subject to problems for staff who would have to volunteer for overtime.
The Labour leader would need to rely on staff volunteering themselves for longer hours on lesser pay.
But with staff being able to earn more through private work, it could be hard to find doctors willing to take on cheap overtime.
Sir Keir said the plans would allow the NHS to provide an extra two million operations, scans and appointments in the first year alone.
It comes as the Conservative party is currently in a pay dispute with junior doctors and consultants over pay, suggesting it may be hard to find staff willing to take on the overtime.
Labour is expected to pledge £1.1billion to pay staff for evening and weekend overtime rates.
Sir Keir, in Liverpool for Labour’s annual conference, said doctors would probably earn more privately but would do NHS overtime as “they want to bring down the waiting list as well”.
But Tory MP Paul Bristow said: “Labour’s plan seems to amount to asking doctors nicely to work weekends. If this is all they have, I would suggest some fresh thinking.
“The only way to cut waiting lists is to continue with the Government’s plan and for union bosses to accept the fair deal on the table.”
The British Medical Association’s Professor Philip Banfield said: “Only once doctors receive restoration of lost relative value will we be in a position to look at the impact that this extra overtime funding may have on waiting lists.”
Christina McAnea, Unison general secretary, added: “This is fine as a stop-gap measure, but this is all it must be. Health workers are already up against it and there are only so many hours in a day.
“But a voluntary scheme where staff are paid fairly, that avoids the use of expensive agencies, makes sense in the short term. The priority must be to retain existing staff and encourage more temporary workers to go on the books and become directly employed NHS employees.
“There must also be a properly funded recruitment campaign to fill thousands of vacancies. That’s how to tackle the huge backlog in the most efficient way.”
