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Boris Johnson has refused to express regret about the timing of lockdown – despite suggestions from a leading scientist that imposing restrictions a week earlier could have halved the coronavirus death toll.

Neil Ferguson said the number of coronavirus infections was doubling every three or four days before lockdown was introduced on March 23 and so moving a week sooner could have saved around 25,000 lives.

But the prime minister insisted he took the “right decisions at the right time” and repeatedly pointed out that Ferguson himself was a member of the government’s scientific advisory group for emergencies (Sage) that advised ministers on when to introduce lockdown.

Ferguson, who was forced to resign from Sage when he broke lockdown rules by allowing a woman to visit him at his London home, told the Commons science committee on Wednesday: “The epidemic was doubling every three to four days before lockdown interventions were introduced.

“So, had we introduced lockdown measures a week earlier, we would have reduced the final death toll by at least a half.”

Ferguson added, however, that based on what was known about transmission and fatalities at the time, the measures were warranted.

Asked about the scientist’s comments at the daily Downing Street briefing, Johnson said: “All such judgments I think will need to be examined in the fullness of time. Professor Ferguson was a member of Sage at the time, which he readily accepts.”

The PM was pressed again on whether he had any regrets again by Sky News political editor Beth Rigby, who stressed as Johnson referred to the government’s scientific advice: “You make the decisions.”

Johnson replied: “We made the decisions at the time on the guidance of Sage including professor Ferguson that we thought were right for this country.

“The questions that are posed are still unanswered and there’s a lot of data we still frankly do not have.

“I know you want me to cast judgement now on everything that happened in the months that have gone by. I just think that of course that moment will come, of course we’ve got to learn lessons, but I just think that it is at this stage premature – there’s still too much that we don’t know.” 

He added later: “All I can say is, at the moment, it is too early to judge ourselves.”

The latest figures from the government show the number of people who have died across the UK after contracting coronavirus has risen by at least 109 in 24 hours. It brings the working total to 40,992.

According to the Office for National Statistics (ONS) the number of people who have died with coronavirus mentioned on their death certificate is now 51,089.

And the number of excess deaths – the number of deaths above the expected number for that time of year – stands at 63,000.