Diane Abbott, the shadow Home Secretary, has written to Ms Patel calling for an independent lawyer-led inquiry into whether the alleged bullying amounted to a breach of the ministerial code. Ms Patel has consistently denied all the allegations.

In the letter, the 90-plus signatories say they have worked or campaigned with her "often at times of stress and high emotion", and add: "We do not recognise the picture that has been painted of her this week.

"There's no argument that she is a tough, assertive and effective leader, but even under extreme pressure she has never crossed a line or lost her temper. She expects high standards, as a strong leader should, but is always professional. She gives her all for this country and deserves far better treatment than she has received."

Instead, the signatories – including Tory peers Lords Rami, Suri and Leigh, election guru Sir Lynton Crosby, Don Porter, the ex-chair of the Conservation Convention, police commissioner Matthew Scott and transplant surgeon Professor Nadey Hakim – say she is "thoughtful, caring and compassionate".

"It is vital for the health of our democracy that good ministers cannot be brought down by smears, gossip and innuendo when they attempt to tackle serious issues in their departments," the writers say. "Far from the unjust treatment she has received this week, the Home Secretary deserves the whole-hearted support of the people of the UK as she attempts to turn around the performance of a vital but notoriously dysfunctional ministry."

Interviewed on ITV's This Morning programme on Thursday, Mr Johnson repeated his support for Ms Patel, saying: "My instinct is very much to 'stick with Prit', as they say. She is doing an outstanding job facing a lot of headwinds. I have made it clear to all my friends in Cabinet that I don't like bullying. But I think Priti is doing an outstanding job."

Cabinet Office sources said the inquiry would investigate what complaints had been made and whether they were appropriately handled at the time. It is expected to report within weeks rather than months, and will report directly to the Prime Minister. It is not expected to be published.

The investigation into whether Ms Patel breached the ministerial code of conduct could rest on a new clause, inserted in January 2018, that that means ministers can be sacked for bullying, harassment or "inappropriate or discriminating behaviour."