A former Tory MP breached the ministerial code by using force against a climate protester at a black-tie City dinner, a government investigation has found.

Mark Field, who stood down from parliament after being suspended as a Foreign Office minister, grabbed a Greenpeace activist, Janet Barker, by the neck and forced her out of the event.

The Cabinet Office investigation found he was “justified in intervening to try to stop” Barker reaching the top table as protesters disrupted the beginning of a speech by Philip Hammond, who was the chancellor at the time.

But the report said that although he had to make a “split-second decision”, he “had the option of simply blocking her way” during the incident in London’s Mansion House in June.

Alternatively, he could have waited for “others to shepherd her out rather than pushing her by the neck out of the room and down the stairs”, the prime minister’s independent adviser on ministerial standards, Sir Alex Allan, wrote.

After reviewing footage of the incident and the comments of interviewees, he found “that the actions Field took, and the force he used, were not consistent with the high standards of behaviour expected of ministers and with treating Barker with consideration and respect. As such it was a breach of the ministerial code.”