At least 95,000 people have entered the UK from overseas since the coronavirus lockdown was imposed, one of the government’s chief scientific advisers has revealed, while repeatedly failing to provide an estimate of how many of these people had Covid-19.

Appearing before MPs on the science and technology committee, Prof John Aston, the chief scientific adviser at the Home Office, admitted that had tougher restrictions been introduced at the border, the peak of the virus may have been delayed – but he did not say by how long, or if this would have saved lives.

Aston, who attends meetings of the Scientific Advisory Group for Emergencies (Sage), which is advising the government during the crisis, was asked repeatedly for the estimated proportion or number of people arriving in the UK with Covid-19.

He insisted instead that a more “robust” assessment was the ratio of imported cases to domestic cases. This model, formulated by Sage, estimates 0.5% of all cases on any given day are imported from overseas.

The government stopped issuing guidance at the border to arrivals from specific countries – including from Italy and China – to self-isolate on 13 March, 10 days before the lockdown was imposed. Since then, there has been little intervention other than advice provided on leaflets and posters. Arrivals will have been subjected to the same lockdown restrictions imposed on the wider population since 23 March.

In a heated exchange Yvette Cooper, the chair of the home affairs select committee, who was appearing as a guest member, asked Aston if ministers had a central estimate of the number of people arriving each week who might have Covid-19 when the decision to stop asking any arrivals to self-isolate was made.

Aston said: “When that [decision] was made, an estimate was made about what the effect of putting further restrictions on the border would be.

“It would delay the epidemic by a small amount of time and therefore it was deemed that wasn’t a sensible thing to do.”

Cooper repeatedly asked for the estimate but it was not provided. She said: “Previously people were asked to self-isolate at the border for 14 days. Inexplicably when other countries were increasing their restrictions or their requirements to self-isolate, the UK lifted them all. It was before the peak in Spain, it was still around the peak in Italy, it was several weeks before the peak in UK.”

Aston’s evidence comes as the government prepares to enforce a 14-day quarantine for arrivals by air at the UK border – a policy that some have suggested would have been more appropriate prior to the UK lockdown on 23 March.

There were 18.1 million arrivals to the UK in the period from 1 January to 23 March across air, land and sea, of whom 273 air passengers were formally quarantined.

Aston told the committee that between 1 April and 26 April there were 95,000 arrivals into the UK by air, of whom about 53,000 were UK citizens.

Defending the use of the modelling that returns an estimate that imported cases make up 0.5% of all domestic cases, he said: “It’s a really complicated modelling thing to do. It was done by Sage. It requires you to understand the prevalence in overseas countries, as well as prevalence in the UK and you need to put that together to get the ratio of incoming cases to domestic cases.”

Aston’s remarks jar with testimony given by Sir Patrick Vallance, the government’s chief scientific adviser, who told the health and social care committee that the UK received a “big influx of cases” from Europe that “seeded right the way across the country”.

The home affairs select committee has repeatedly requested the government publishes the scientific advice provided by Sage that informed its border policy.

Stephen Doughty, a shadow minister and former member of the committee, who has been pursuing the issue of border policy during the crisis, said: “Simply claiming it’s all a bit complicated and refusing to publish the models on which these critical decisions were based is not good enough.

“We need to see urgent publication of the models and scientific advice that led to the Cabinet deciding not to act when the pandemic was raging. The revelation that even since the lockdown likely more than 100,000 have entered with scant checks, also begs the urgent question – why has the government still not introduced measures?”