Theresa May's attacks on Boris Johnson are a departure from convention

Personal misgiving could be at the heart of the former British prime minister's criticisms of her successor in Parliament

Boris Johnson, left, served as foreign minister in former British prime minister Theresa May's cabinet. AP Photo

There is a longstanding convention in British politics that, when prime ministers stand down from office, they retire from the limelight and do not publicly criticise policies adopted by their successors.

From Margaret Thatcher through to David Cameron, the main activity of departed prime ministers has been to concentrate their energies on writing their memoirs – with varying degrees of literary success – thereby seeking to define their legacy.

The only notable exception to this convention was Edward Heath who, having lost office in 1974, never came to terms with the fact that Thatcher, whom he believed had been less than supportive of his government, had replaced him as leader of the Conservative Party. For the entire period of her tenure in Downing Street, from 1979 to 1990, Heath remained a stubborn and vociferous critic of her policies from his seat in the House of Commons, an attitude derogatively referred to by his Tory colleagues as “the great sulk”.

Margaret Thatcher and Edward Heath, left, supposedly did not get on very well despite being from the same political party. Getty Images

Recent attacks by Theresa May on the approach being adopted by Boris Johnson, her successor and fellow Conservative, have therefore raised the prospect of another Heath-like stand-off.

When Mrs May was forced to stand down from office a year ago over her bungled handling of the Brexit negotiations, there was a general consensus that she would simply retire to her Oxfordshire home to write her memoirs and devote her time to local constituency issues. After nearly 20 years of holding a succession of senior positions, from chairman of the Conservative party in 2002 through to her appointment as prime minister in 2016, most of her colleagues believed she deserved her well-earned retirement.

Yet, going by her increasingly critical tone towards Mr Johnson, she clearly does not believe that just because she is no longer a Downing Street resident, her political career has drawn to a close. On the contrary, in recent months Mrs May has used her position as a backbench MP to attack the Prime Minister's handling of the Brexit negotiations with the European Union as well as his government’s less-than-impressive handling of the coronavirus pandemic.

Most recently, Mrs May criticised the Prime Minister for appointing a trusted confidant as national security adviser last month.

Since its inception under David Cameron in 2010, the National Security Council (NSC) has been given responsibility for safeguarding the country's interests. They concern overseeing military operations overseas, such as the recent campaigns in Afghanistan and against ISIS, as well as deliberating over whether the Chinese telecoms giant Huawei should be allowed to participate in the construction of Britain's new 5G network.

One of the reasons for Mark Sedwill's departure as NSA could be the support he received from Theresa May. Reuters

During her time as home secretary and then prime minister, Mrs May regularly attended NSC meetings and was responsible for the appointment of Mark Sedwill, the organisation's current head. Consequently, she was deeply alarmed when Mr Johnson announced last weekend that he was appointing David Frost, his chief Brexit negotiator, to replace Mr Sedwill.

For several months, Mr Johnson has been keen to replace Mr Sedwill – who previously served as Nato’s ambassador to Afghanistan – following concerns that, by holding the positions of head of Britain's civil service and cabinet secretary in addition to his national security responsibilities, he exerted too much influence in government.

Indeed, the fact that he had become the most powerful civil servant of modern times was due in no small measure to his close relationship with Mrs May, who regarded him as a trusted ally and promoted him to all three roles. In such circumstances, therefore, it was hardly surprising that Mr Johnson should want to replace an official who was so closely associated with his predecessor’s administration.

David Frost will take over from Mark Sedwill as the UK's national security adviser. AFP

This did not prevent Mrs May from launching a blistering attack on Mr Johnson in Parliament this week.

"I served on the National Security Council for nine years – six years as home secretary and three as prime minister," Mrs May said. "During that time, I listened to the expert independent advice from national security advisers. Why, then, is the new national security adviser a political appointee with no proven experience in national security?”

In response, Michael Gove, the Cabinet Office Minister, pointed out – with some justification – that not all the previous incumbents had been steeped in national security issues before their appointments, and that, just like Mr Frost, they had enjoyed distinguished diplomatic careers prior to taking over the national security role.

Mrs May may have been correct to question Mr Johnson’s high-handed treatment of Mr Sedwill. But her intervention demonstrates that her antipathy towards Mr Johnson, which dates back to her time in office, has not abated.

Then British foreign secretary Boris Johnson, left, and former prime minister Theresa May at the UN headquarters in New York in September 2017. AP Photo

Mrs May's appointment as prime minister in 2016 was mainly due to the failure of Mr Johnson – who was then the front-runner for the job – to muster an effective campaign strategy. Consequently, when she won the Tory ballot she was obliged to give him a senior position in her government, appointing him foreign secretary. Even so, she never fully trusted him, and kept him at arm's length from the inner sanctum of her government. Her suspicions about Mr Johnson's loyalty ultimately proved to be correct when he resigned his post in 2018 in protest at the Brexit deal Mrs May had negotiated with the EU.

So far as Britain’s national security is concerned, it is now up to Mr Frost to demonstrate that Mr Johnson has made the right call. But the willingness of Mrs May to publicly lambast her successor over the decision suggests that, unlike most previous occupants of Downing Street, the former prime minister has little intention of slipping quietly into the sunset of her career.

Con Coughlin is the Telegraph’s defence and foreign affairs editor