British Prime Minister Boris Johnson on Wednesday readied a fresh cabinet "to build back better from the pandemic" in a reshuffle that cast Foreign Secretary Dominic Raab's future into doubt.

Raab, who deputised as premier last April when Johnson was in intensive care with Covid-19, has faced sustained criticism for his handling of the Afghanistan crisis.

The spotlight was also on Education Secretary Gavin Williamson, who like Raab was notably absent from the government front bench of the House of Commons at Johnson's weekly question and answer session.

The gaffe-prone Williamson was the first minister to indicate he was out of a job, tweeting that it had been a "privilege" to serve in his role since 2019 when Johnson took office on a vow to "get Brexit done".

Now that Britain has formally left the European Union after a years-long divorce, the Conservative government has been battling to tame one of the world's highest per capita death tolls from the pandemic.

Johnson on Tuesday outlined a plan to restore restrictions if needed, to avert fresh crisis for the state-run National Health Service heading into what could be a difficult winter with hospitalisations rising again.

After days of speculation, a Downing Street source confirmed the reshuffle was happening, saying Johnson would "put in place a strong and united team to build back better from the pandemic".

"The PM will be appointing ministers this afternoon with a focus on uniting and levelling up the whole country," the source added.

Vacationing as Kabul fell

Raab was on holiday on a Greek island as the government scrambled to evacuate British nationals and Afghan staff as the Taliban advanced and seized power last month.

Trade Secretary Liz Truss, who has brokered a series of deals since Britain's full exit from the EU in January, has been tipped to replace Raab.

Williamson meanwhile has faced a barrage of complaints about his handling of school closures, examination arrangements and university admissions during the coronavirus emergency.

Brexit point man Michael Gove has been suggested as a replacement for Home Secretary Priti Patel, another arch Brexiteer who has been under pressure due to a surge in migrant crossings from France, along with bullying claims.

However, Patel sat alongside Johnson in the Commons on Wednesday, suggesting she may be safe.

Finance minister Rishi Sunak was another flanking the prime minister, having emerged as a Conservative darling during the pandemic.

But Justice Secretary Robert Buckland and Housing Secretary Robert Jenrick confirmed on Twitter that they were on the way out.

"On to the next adventure," Buckland said.

As Johnson met with colleagues in his office inside the Commons, away from the public glare of 10 Downing Street, his official spokesman said the premier understood "the importance of having a diverse cabinet".

But the spokesman would not guarantee that female representation in the ministerial team would be maintained at its current high level.

Political strategist Dominic Cummings, the Brexit mastermind who fell out with Johnson last year, branded the shake-up the "Carrie reshuffle", suggesting that allies of the prime minister's wife will be rewarded.

The Downing Street spokesman denied this.