Sir Gavin Williamson should apologise to MPs for bullying a former chief whip after he was not allocated tickets to the late Queen’s funeral, Parliament’s watchdog has said.

He was forced to quit as a Cabinet Office minister just days into Rishi Sunak’s premiership last year over a series of expletive-laden messages to Wendy Morton.

The texts amounted to “offensive and intimidating behaviour”, Westminster’s Independent Expert Panel (IEP) said on Monday, as it concluded that Sir Gavin’s conduct was “an abuse of power, finding that it had gone beyond vigorous complaint or political disagreement to a threat to lever his power and authority as a former chief whip to undermine Ms Morton personally”.

The IEP said it had “considered carefully” whether he should face suspension from the Commons but had instead decided a “full and unreserved apology” was required.

Sir Gavin has accepted the IEP’s ruling that he should make a personal statement in the House of Commons and undergo “appropriate behaviour training” to increase his awareness of the impact of bullying on others, the panel said.

The IEP’s report said Sir Gavin sent Ms Morton text messages after he did not receive an invitation to the funeral, which he attributed to his having not supported then-prime minister Liz Truss in the recent leadership election.

The exchange concluded with him saying: “Well, let’s see how many more times you f*** us all over. There is a price for everything.”

Sir Gavin, who had already been twice sacked from the Cabinet in disgrace, bowed to pressure to resign as minister without portfolio last November after a number of allegations piled up against him.

As well as Ms Morton’s complaint, which related to Sir Gavin’s time as a backbench MP, he was alleged to have bullied a former official at the Ministry of Defence by telling them to “slit your throat” and was also accused of “unethical and immoral” behaviour when he was Tory chief whip.

Ms Morton complained to the Independent Complaints and Grievance Scheme, the parliamentary watchdog responsible for investigating complaints of inappropriate behaviour against MPs.

While Parliamentary Commissioner for Standards Daniel Greenberg had cleared Sir Gavin of the charge that his conduct amounted to bullying or harassment, the IEP upheld an appeal against the decision by Ms Morton.

The IEP’s report said: “In the clearest terms, he was going to make her position difficult and frustrate her role as the newly appointed chief whip, all as revenge for his perception that she had denied him (and others) a ticket to the Queen’s funeral because he was not a prime minister Truss supporter.

“This was not just a party matter or a simple political difference. It was direct action by way of threat to her personally.

“And that is clearly how she felt it.”

Sir Gavin, who was knighted after being nominated for the honour by Boris Johnson last year, is a divisive figure at Westminster, where he is viewed with suspicion by many Tory MPs because of his reputation as an inveterate plotter.

He was first sacked by former prime minister Theresa May as defence secretary in 2019 for leaking details of a National Security Council meeting, and again by Mr Johnson as education secretary over the Covid-19 A-levels debacle.

However, he was a key ally of Mr Sunak, whose judgment in appointing him to his cabinet was criticised when Sir Gavin was forced to quit not long afterwards.

Other ministerial departures following to complaints about their conduct during the Mr Sunak’s tenure as Prime Minister have included former Conservative Party chairman Nadhim Zahawi and former justice secretary Dominic Raab.