Doctors have warned that plans to transform the NHS into a seven-day service are a waste of vital resources that could threaten the quality of care during the week and prove a danger to patient safety.

In a strongly worded attack, the British Medical Association accuses NHS England of pushing ahead with an “unfunded, undefined” strategy that is “wholly unrealistic” given the health service’s chronic staffing and financial problems.

Offering a full range of NHS services at weekends is “neither desirable nor feasible”, not needed in order to meet medical need and unlikely to attract enough patients to justify such a huge change, the doctors’ union says. Putting on outpatient clinics and performing routine operations on Saturdays and Sundays will inevitably lead to some of those services being cancelled during the week, it adds.

The BMA’s broadside against the plans is contained in a detailed submission it has made to the Doctors and Dentists Review Body (DDRB), which advises ministers on doctors’ salaries, for its inquiry into what medics should be paid in a seven-day NHS.

“Ill thought through implementation could threaten both the standard of service patients receive and the viability of the service … We believe that pursuing an ill-informed seven-day service model would be at best wasteful for the NHS and at worst threaten patient safety,” the BMA says in the executive summary of its evidence, which it published on Friday .

NHS England has pledged to start offering a wide range of services on all seven days of the week in order to improve the care patients receive at weekends and tackle the high death rates that occur on Saturdays and Sundays, and also to better fit in with patients’ working lives.

When he made the high-profile commitment in October 2013, Prof Sir Bruce Keogh, NHS England’s national medical director, said the dramatic expansion of its role at the weekend would cover much more than simply emergency care – A&E care and urgent surgery – which is already close to a 24/7 service in many places.

“It is also about access to doctors, diagnostics and elective operations so that people do not have to miss work to get healthcare. If someone needs a day-case operation, why can’t they have that on Saturday, recover on Sunday and be back to work on Monday?” he said at the time.

Keogh wants hospitals to have access to x-rays, ultrasound scans and emergency general surgeries at weekends and believes that mental health and community care services also need to be available 24/7 if the expanded role is to work.

NHS England decided to act after research studies showed that patients were 16% more likely to die if they were admitted to hospital on a Sunday and 11% more likely to die if they were admitted on a Saturday. It also faced growing pressure to end the relative lack of diagnostic and treatment services at weekends.

But the BMA questions if the funding is available to make the plans work. “The BMA believes that without significant resource investment, a seven-day service will not be possible without compromising and/or re-evaluating existing service provision in the NHS. There is a real risk that patient safety could be compromised in the week if the government attempts to implement seven-day services without additional resource investment.”

In its evidence the BMA accepts the principle that patients should receive the same quality of care regardless of which day they start their treatment. But it criticises NHS England for refusing to specify which services it intends to start providing at weekends, how much such a huge shift will cost and how that will be staffed.

“The BMA believes a properly costed model for seven-day services is a prerequisite for implementation, otherwise the model risks being unsustainable for the NHS”, it says.

In an interview with the Guardian, Dr Mark Porter, the BMA’s leader, warned that patients would be unable to access certain forms of NHS care on some weekdays as a result of those services also becoming available at the weekends.

“If we’re going to transfer resources to the weekend, we should know what services we are stopping doing during the week in order to do that. If the decision is taken [by a hospital] to put half a dozen outpatient clinics on a Sunday afternoon then they have to be taken from somewhere else – that is, they won’t be on on a particular day during the week. That’s the inescapable reality of doing this within existing resources, but politicians’ vision doesn’t go into that sort of detail,” Porter said. There would be a “diversion of service” rather than an extension of service.

Ministers seemed to believe that the shift into full seven-day operation would not need any extra money, despite the NHS’s 1.3m-strong workforce already being very stretched, Porter said. “It’s not realistic to carry on doing what we are doing at the moment and then add a new service on top of it within existing resources. More money is vital.”

He also criticised NHS bosses for the lack of evidence that significant numbers of patients want to come into hospital at the weekend for a diagnostic test or planned operation. Likely demand should be assessed before the planned expansion of weekend services occurs, he said.

He suggested NHS England announced the move to a seven-day service as a result of pressure from the government and called it “a headline” and “a slogan”.

NHS leaders privately believe the BMA, despite its support for the principle of seven-day services, is trying to protect doctors from having to work more at weekends and overnight. The union contends that most consultants already work at those antisocial times, with some – such as those in emergency departments – doing so regularly.

NHS England declined to respond to the BMA’s concerns directly. A spokeswoman said 2015 would see “significant progress” towards the goal of a seven-day service. “NHS England is the only healthcare system in the world that is trying to sort out the issue of seven-day-a-week services. Individual hospitals internationally have done some work on it, but never a national health service.

“Over the last year, local NHS trusts have been making good progress in developing their understanding as to how the service will work at a local level. During 2015, we expect to see significant progress towards making seven-day-a-week services a reality as we owe it to our patients to ensure it happens.”

The Department of Health said: “Patients expect to receive the same standard of care regardless of the day of the week. A seven-day NHS service will speed up diagnosis and discharge times as well as reducing the amount of time patients need to spend in hospitals at weekends . This is why we’ve asked independent pay experts [the DDRB] to advise on how employment contracts could be changed to make seven day services a reality.”