Junior doctors in the West Midlands have refused to go back to work despite an order from their NHS trust, as thousands of medics strike across England.
Sandwell General Hospital in West Bromwich has declared a "level 4" major incident and said its junior doctors must return to the wards to relieve the pressure.
But some doctors have accused the hospital of pre-planning the incident after a letter from the NHS trust's medical director was sent to them on Tuesday morning, but was dated Monday.
The letter from Dr Roger Stedman, medical director at Sandwell and West Birmingham Hospitals NHS Trust, said: "Sandwell General Hospital is now at escalation level 4 and is reporting an internal incident".
It said "all junior doctors providing ward-based care need to attend work", adding: "Over the past few days there has been a surge in activity which has continued today.
"There are over 50 additional adult beds open with further medical outliers in surgery.
"There have not been enough discharges today to accommodate patients requiring admission.
"Tomorrow we will be running Sandwell General Hospital site as a Trust internal incident with all medical teams working to increase the numbers of discharges.
"I have reviewed the situation this afternoon and I believe that given the situation and staffing levels, it would be unsafe to deliver care to all our in-patients with a reduced workforce."
It came as the first strike by junior doctors in 40 years takes place in a row over a new contract.
Around 100 picket lines have been put in place, with a large concentration in and around London.
A spokesman for the BMA said doctors in Sandwell should continue to strike until further notice.
He said: "Junior doctors should continue with industrial action until NHS England has confirmed, and the BMA has agreed - via the agreed escalation process - that a major unpredictable incident is taking place for a specific trust.
"The BMA will notify members as soon as such an incident is in place."
Doctors remain on the picket line at Sandwell.
Bridget Riley, who works in the gastroenterology unit at the hospital, said there was an "internal capacity incident" at the trust, which "means we're under more pressure than normal".
She said there had been several similar incidents in recent weeks due to winter pressures on the NHS.
Ms Riley told Sky News that doctors should only be ordered to work if the incident was "unpredictable", rather than something that was known about on Monday.
She added: "Patient care is at the centre of what we do but this strike has been known about.
"All the consultants have been asked to come in, many who support junior doctors. Elective procedures have been cancelled so really this is no different to Sunday, should you come into hospital on a Sunday."
Asked what it would take to get her back to work, she said: "If there became a concern about patient safety, we would come back to work.
"On Christmas Day, we're under a lot of pressure but we don't get phone calls from medical staffing asking us to come in from home."
Another doctor told the BBC: "If this was a major incident the trust has been facing as per the letter 'over the past few days', why wait until now?"
NHS England said it is reviewing the situation and supporting the trust.
The BMA is being backed in its action by other unions, including Unite, Unison and the RMT, whose members helped bolster numbers on some picket lines.
Junior doctors - the term for doctors working below consultant level - represent a third of the medical workforce and just over 37,000 of more than 50,000 junior doctors are members of the BMA.
Despite last-ditch talks to prevent the strike, around 4,000 operations and procedures have been cancelled, with thousands more routine appointments also postponed.
Patients have been told hospitals are under pressure and asked to attend A&E only if they have a genuine emergency.
The basis for the current round of negotiations is the Government's offer from early November, including an 11% rise in basic pay for junior doctors.
This is offset by plans to cut the number of hours on a weekend for which junior doctors can claim extra pay for unsocial hours.
In a message on Facebook, Labour leader Jeremy Corbyn showed his support, saying: "No NHS worker takes lightly the decision to strike, but the blame must be laid at the door of this Government for the way it has treated doctors and now seeks to smear them in the press.
"It is time for this Government to apologise to junior doctors and negotiate a fair deal that gets our NHS working again."