Boris Johnson on Wednesday apologised for "the pain and the loss and the suffering" caused by the Covid-19 pandemic, as he gave evidence at a public inquiry into his government's handling of the global health crisis.

The former prime minister, who has faced a barrage of criticism from former aides for alleged indecisiveness and a lack of scientific understanding during the pandemic, is facing two gruelling days in the witness box.

Johnson – forced from office last year over lockdown-breaching parties held in Downing Street during the pandemic – accepted that "mistakes" had "unquestionably" been made but repeatedly insisted he and officials did their "level best."

"I understand the feeling of the victims and their families and I'm deeply sorry for the pain and the loss and the suffering to those victims and their families," Johnson said.

Johnson, 59, was briefly interrupted as a protester was ordered from the inquiry room after refusing to sit down during the apology.

Several others were also later removed.

"Inevitably we got some things wrong," Johnson continued, adding he took personal responsibility for all the decisions made.

"At the time I felt... we were doing our best in very difficult circumstances."

The former premier arrived around three hours early for the proceedings, with some suggesting he was eager to avoid relatives of the Covid bereaved, who gathered outside later in the morning.

Nearly 130,000 people died with Covid in the UK by mid-July 2021, one of the worst official per capita tolls among Western nations.

Johnson – whose lengthy written submission to the inquiry will be published later on Wednesday – insisted the "overwhelming priority" of his government had been protecting the National Health Service (NHS) and saving lives.

Rebutting evidence Britain fared worse than European neighbours, he argued "every country struggled with a new pandemic" while noting the UK had an "extremely elderly population" and is one of the continent's most densely populated countries.

Johnson, who was treated in intensive care for Covid early on in the pandemic, has reportedly spent weeks with his lawyers, reviewing thousands of pages of evidence ahead of his testimony.

His grilling began with questions about a failure to provide about 5,000 WhatsApp messages on his phone from late January 2020 to June 2020.

"I don't know the exact reason," he claimed, adding the app had "somehow" automatically erased its chat history from that period.

Asked if he had initiated a so-called factory reset Johnson said: "I don't remember any such thing."

Johnson's understanding of specialist advice was doubted last month by his former chief scientific officer, Patrick Vallance, who said he was frequently "bamboozled" by data.

The ex-leader has also denied claims he said he would rather "let the bodies pile high" than impose another lockdown.

Inquiry counsel Hugo Keith questioned Johnson about Downing Street dysfunction during the pandemic, and claims of general incompetence by numerous senior officials.

"What all those comments reflect is the deep anxiety of a group of people doing their level best who cannot see an easy solution and are naturally self-critical and critical of others," Johnson insisted.

His former top aide Dominic Cummings and communications chief Lee Cain both criticised their ex-boss when they gave evidence at the inquiry.

Cummings, who has faced his own criticism for writing expletive-filled WhatsApp messages, said Johnson circulated a video to his scientific advisers of "a guy blowing a special hairdryer up his nose 'to kill Covid'."

Cain said Covid was the "wrong crisis" for his ex-boss's skillset, adding that he became "exhausted" by his alleged indecision in dealing with the crisis.

Prime Minister Rishi Sunak, who was Johnson's finance minister during the pandemic, is due to be questioned at the inquiry in the coming weeks. (AFP)