A mental health trust has launched an investigation after a heavily pregnant patient was allegedly dragged from a seat and pinned to the floor by staff.
The woman was knocked face down to the floor by a male nurse in a psychiatric hospital, according to a witness.
Three members of staff are said to have then held her down.
The Central and North West London NHS trust said a member of staff had been removed from clinical duties while the incident was investigated.
The alleged incident occurred on the night of 10 July while a former adviser to the health secretary on patient safety was an inpatient at the unit.
The former adviser, Alison Cameron, said that while the woman, who was eight and a half months pregnant, was being verbally aggressive she hadn't made any physical threats.
"A male nurse came marching over from the treatment room to the dayroom," said Ms Cameron.
"He was threatening the woman saying 'If I hear your voice again, I will, I will...' and without finishing his sentence, he physically manhandled the woman from her seat to the floor. Three other members of staff then pinned her down."
Ms Cameron, who has been a patient advocate for years, said the use of force was wholly unnecessary. "I know safe restraint, and this wasn't it."
She has reported her concerns to the Care Quality Commission (CQC).
The trust has asked the BBC not to name the hospital to protect the identity of the patient, who remains in the unit.
Some mental health patients can, on occasion, be physically restrained by staff though many hospitals are trying to stop the practice.
Ms Cameron, an associate at the King's Fund and an expert on patient safety, says her experience of spending two nights at the unit last weekend was appalling.
She suffers from PTSD and needed treatment after feeling suicidal. Having spent 12 hours in an accident and emergency unit waiting for a bed in a psychiatric hospital, she was taken to the hospital on Saturday afternoon.
"It was far from therapeutic. I noted from the outset a lot of what I would describe as manhandling of patients which appeared to me to be the accepted culture on the ward," she said.
The ward was short-staffed, with lots of bank and agency nurses, said Ms Cameron.
People were expected to sleep in dormitories, but because of the noise that some very sick patients were making, rest was impossible and she said some patients felt unsafe.
"There was constant shouting and swearing, everyone was tense and tired, there was potential aggression from patients, and the staff were very demoralised. I needed peace and quiet, and rest, to help me recover and I could see that wasn't possible."
The atmosphere deteriorated at night on the ward, Ms Cameron said, which is when the pregnant woman was allegedly assaulted.
In a statement, the Central and North West London NHS trust said it acted as soon as it learned of the allegation.
"The chief executive immediately asked for an investigation which is well underway; witnesses (staff and patients) are being interviewed to find out what happened. We want to know why restraint was necessary, because it is always an extreme event, even when occasionally needed.
"After the incident a doctor examined the woman in question and she was fine. We are working with her and her family about the incident and helping her from here.
"We're keeping the CQC informed. Until the investigation is completed, the member of staff involved has been taken off clinical duties which is a normal - and neutral - step following this type of allegation."