A former Tory MP who grabbed a female climate change protester around the neck has been found to have breached the ministerial code – five months after Boris Johnson dropped an investigation.

Mark Field was able to quietly leave the Commons at the general election after the incoming prime minister decided a full probe into his behaviour was no longer needed.

Just weeks later, a Cabinet Office inquiry under the code has concluded his manhandling of the Greenpeace activist flouted “the high standards of behaviour expected of ministers”.

Mr Field was caught on camera holding Janet Barker around the throat after she joined a protest that disrupted a black-tie dinner in the City of London.

But Alun Cairns – who was forced to quit as Welsh secretary as the campaign started, after being accused of lying about an aide’s sabotaging of a rape trial – has been cleared of breaching the ministerial code.

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He claimed he was unaware of the role played by his former member of staff, Ross England, in the collapse of the trial until after the story broke.

Mr Cairns was accused of “brazenly lying” after BBC Wales said it had obtained a leaked email which suggested he had been made aware of the allegations as early as August last year.

However, the report by Alex Allan, the prime minister’s independent adviser on ministers’ interests, concluded he did not know the details until they become public.

Despite the controversy, Mr Cairns increased his majority in his South Wales seat last week – but has been replaced as Welsh secretary by Simon Hart.

In July, when Mr Johnson sacked Mr Field as a foreign office minister, a No 10 spokesperson said: “Mark Field has now left the government.

“The prime minister considers this issue was a matter for the previous prime minister ... as a minister under her appointment.”

In relation to that inquiry, Sir Alex found the departed MP “was justified in intervening” to try to stop Ms Barker at the high-profile dinner.

But he added that “the actions Mr Field took, and the force he used, were not consistent with the high standards of behaviour expected of ministers and with treating Ms Barker with consideration and respect”.

The report concluded: “As such it was a breach of the ministerial code.”

After the incident, Ms Barker said Mr Field had pushed her so hard as they reached the door of the room that she had almost fallen.

Calling for him to take anger management classes, she added: “I want him to think about what he did, why he did it and address his behaviour.”