Boris Johnson has been accused of spending too little time on protecting Britain’s national security in a highly critical letter sent directly to Downing Street by the chair of a cross-party watchdog.

The chair of the national security committee, the former Labour foreign secretary Dame Margaret Beckett, writes: “We are deeply troubled by the persistent signs that our nation’s security is no longer a priority for the government”.

The former minister raises concerns about the fallout from the withdrawal from Afghanistan – and complains to the prime minister that there has been “a significant reduction in your personal engagement” compared with his predecessors David Cameron and Theresa May.

Accusing Johnson of “a more relaxed approach to national security”, Beckett writes that the prime minister was now expected to chair only 12 meetings of the national security council a year, after a recent internal revamp.

That contrasted with about 35 meetings a year under the old system created by Cameron, in which the powerful committee, composed of senior ministers plus military and intelligence officials, met weekly whenever parliament was sitting to agree a common response to the key security issues of the day.

“While we understand the pressing need to rebuild the economy after the pandemic, we fear that complacency may be creeping in,” Beckett writes in the letter, on behalf of the 22-member body of MPs and peers.

Though chaired by Beckett, the committee’s members also include several senior Conservative backbenchers, including Tom Tugendhat and Tobias Ellwood, the chairs of the foreign affairs and defence select committees.

On Afghanistan, Beckett said committee members said they had been struck by “the apparent complacency and lack of urgency” across Johnson’s government “in the wake of a disastrous experience for the UK”.

The prime minister has repeatedly ruled out holding a public inquiry into the chaotic withdrawal from Kabul.

There was a complaint from the chair that the UK had “acquiesced to US strategic thinking on Afghanistan for too long” – including both former president Donald Trump’s decision to strike a deal with the Taliban and Joe Biden’s final announcement to withdraw troops in the summer.

The messy exit, an emergency airlift supervised by US, British and other troops, suggested “a systemic failure by the government to prepare properly for a scenario that it had, in fact, foreseen”, Beckett says, highlighting again the recent devastating testimony of the Foreign Office whistleblower Raphael Marshall.

The committee chair asks whether the Foreign Office’s “crisis response capabilities are fit for purpose”, given how many Afghans desperate to be evacuated were left behind and why the Home Office “still cannot commit” to launching the planned Afghan citizens resettlement scheme by March 2022.

There was also a complaint about the “relegation of national security as a spending priority” in the last two spending reviews. Defence and the Foreign Office will receive “the smallest annual, real-terms growth” of all government departments, while cuts to the aid budget amount to £5bn a year until 2023-24 at the earliest.

In her conclusions, Beckett calls on Johnson to spend more time on national security, by chairing the national security council “at least once per fortnight”.

To support her argument, she quoted Britain’s wartime leader: “Sir Winston Churchill once said: ‘I never “worry” about action, but only about inaction.’”