The Government's promise of bringing in a truly seven-day NHS service cannot be achieved with current funding, an expert has said.
Professor Julian Bion, from the University of Birmingham, who is carrying out a large review into seven-day working, said the health service was "20 years" away from achieving a seven-day service.
Health Secretary Jeremy Hunt used his drive to create a seven-day NHS as one of the main reasons for reforming junior doctors' contracts.
He relied on a study involving NHS medical director Sir Bruce Keogh which found more patients die if they are admitted to hospitals at weekends. Mr Hunt has argued there is a need to bring down these "excess deaths".
Professor Bion said there had been a focus on mortality but this was actually a relatively small effect. He said there needed to be more focus on problems for patients admitted at weekends, such as complication rates.
He added: "I'm convinced seven-day services cannot be achieved within current funding.
"There are huge gaps. I think we're 20 years away from actually being able to achieve a seven-day service given the current challenges but I would love to be wrong."
He said he was "reasonably confident" that care is not as good at the weekends "which translates to less good outcomes for patients".
And he argued patients should not be put in the "deep freeze" over the weekend until services resume on Monday morning.
Rachel Meacock, health economist at the University of Manchester, said some of the arguments around what causes the so-called "weekend effect", with patients suffering higher death rates, were "flawed".
She added: "There is absolutely no causal evidence that consultant staffing levels are causing the weekend effect."
She said the death rate was higher for patients admitted at weekends, but the total number of deaths at weekends was lower because there are fewer admissions at weekends.
Calling for the Government to "pause" its policy, which has "got ahead of the evidence", she added: "There is no evidence to support the move to seven-day services, there is no evidence of what is going to happen if we divert our resources away from the week to weekends. We don't know what is going to happen to patient outcomes and we don't know what is going to happen to costs."
Professor Paul Aylin, of Imperial College London, said it was "justifiable" to look at why the weekend effect is happening and make changes.
He said it was not true that there was no problem with quality of care at the weekend.
A Department of Health spokesman said: "The established consensus from eight studies in the last six years is that the 'weekend effect' exists, so the Government makes no apology for acting on our ambition to create a safer, seven-day NHS.
"We have a clear plan for the future - by March next year a quarter of the population will receive care that meets the clinical standards that have been prioritised for delivering seven-day services in hospitals."