The prime minister’s chief political adviser, Dominic Cummings, and a data scientist he worked with on the Vote Leave campaign for Brexit are on the secret scientific group advising the government on the coronavirus pandemic, according to a list leaked to the Guardian.

It reveals that both Cummings and Ben Warner were among 23 attendees present at a crucial convening of the Scientific Advisory Group for Emergencies (Sage) on 23 March, the day Boris Johnson announced a nationwide lockdown in a televised address.

Who's who on secret scientific group advising UK government? Read more

Multiple attendees of Sage told the Guardian that both Cummings and Warner had been taking part in meetings of the group as far back as February. The inclusion of Downing Street advisers on Sage will raise questions about the independence of its scientific advice.

There has been growing pressure on Downing Street in recent days to disclose more details about the group, which provides scientific advice to the upper echelons of government during emergencies. Both the membership of Sage and its advice to ministers on the Covid-19 outbreak is being kept secret.

Warner, a data scientist, was reportedly recruited to Downing Street last year by Cummings after running the Conservative party’s general election campaign model. He is also said to have worked closely with Cummings on the data modelling used in the Vote Leave campaign for the UK to leave the European Union.

The government’s former chief scientific adviser Sir David King said he was “shocked” to discover there were political advisers on Sage. “If you are giving science advice, your advice should be free of any political bias,” he said. “That is just so critically important.”

Told that Cummings was in the 23 March meeting, King replied: “Oh my goodness. Isn’t this maybe why they don’t want us to know who was there?”

King said political advisers were never on the equivalent committees of Sage when he chaired them and argued that Cummings, who is not a scientist, could report his own interpretation of Sage advice back to the prime minister.

Other former members of Sage also said they could not recall political appointees being on previous committees. David Lidington, a former Cabinet Office minister and de facto deputy to Theresa May when she was prime minister, said: “I’m not aware of any minister or special adviser, certainly not in Theresa May’s time, ever having been involved in the scientific advisory panels.”

In a statement provided by Downing Street, a government spokesperson said: “Expert participants often vary for each meeting according to which expertise is required. A number of representatives from government departments and No 10 attend also.”

Late on Friday, Downing Street released a second statement. “It is not true that Mr Cummings or Dr Warner are ‘on’ or members of Sage. Mr Cummings and Dr Warner have attended some Sage meetings and listen to some meetings now they are all virtual. Occasionally they ask questions or offer help when scientists mention problems in Whitehall,” a No 10 spokesman said.

“Sage provides independent scientific advice to the government. Political advisers have no role in this,” the spokesperson added. “Public confidence in the media has collapsed during this emergency partly because of ludicrous stories such as this.”

Downing Street declined to say how many Sage meetings Cummings and Warner attended, or whether any other political advisers took part.

Sage participants told the Guardian the Downing Street advisers were not merely observing the advisory meetings, but actively participating in discussions about the formation of advice.

In a letter to parliament this month, Sir Patrick Vallance, the UK government’s chief scientific adviser, who chairs Sage, said the “decision not to disclose” membership of the committee was based on advice from the Centre for the Protection of National Infrastructure.

“This contributes towards safeguarding individual members’ personal security and protects them from lobbying and other forms of unwanted influence which may hinder their ability to give impartial advice,” Vallance wrote. “Of course, we do not stop individuals from revealing that they have attended Sage.”

On Friday, England’s chief medical officer, Prof Chris Whitty, appeared to signal a change in direction, saying the public had a right to know who sat on Sage. He told a Commons science select committee that while it was important to consider security concerns, there was “absolutely no barrier” from him or Vallance. Asked if in the current climate revealing the names of who was on the group would boost public confidence in the scientific advice being given, Whitty replied: “Yes.”

Several members on Sage, as well as scientists on its advisory subcommittees, are known to be frustrated at what they view as a culture of secrecy that risks straining public trust in the government’s response to Covid-19.

Since the outbreak, ministers have stuck to the script that their policies are guided by scientific advice, while declining to reveal where the advice is coming from or what exactly it contains.

Other countries have been more open about the scientific evidence behind their decisions, and the UK’s approach has raised eyebrows overseas.

The New York Times reported on Thursday that Sage operates in “a virtual black box”. “Its list of members is secret, its meetings are closed, its recommendations are private and the minutes of its deliberations are published much later, if at all.”

The Guardian understands that Sage first met for a precautionary meeting to discuss Covid-19 on 22 January, then again on 28 January. It met a further nine times in February, and 10 times in March. It is currently meeting around twice a week.

It understood that while the chief medical officers and chief scientific advisers of the devolved administrations in Scotland, Wales and Northern Ireland have been allowed to listen in on Sage meetings, they have been doing so as observers. Unlike Cummings and Warner, they were not allowed to ask questions, having to instead submit them in writing in advance.

While core members of the committee, such as Whitty, attend all meetings, other clinical experts, scientists and epidemiologists do not attend every meeting, but can be asked in on a rotating basis to provide specific advice. Sage tends to be guided by specific questions that they are asked to consider by the Cabinet Office’s emergency Cobra meetings.

Other Sage participants at the 23 March meeting included Sharon Peacock, the director of the National Infection Service at Public Health England, and Ian Diamond, the head of the Government Statistical Service. Neil Ferguson, the Imperial College epidemiologist whose models have been central to government decision–making, was also present, along with fellow infectious disease specialists, Graham Medley and John Edmunds.

Others attendees included Brooke Rogers, a professor of behavioural science at King’s College – who also chairs the Cabinet Office’s National Risk Assessment Behavioural Science Advisory Group – and James Rubin, also at King’s, who chairs a Sage subcommittee that provides specific advice on behavioural science.

However it is the inclusion of two Downing Street political advisers that will raise questions over whether the structure of the government’s scientific advisory process is free from political interference.

A source in Downing Street said that in March Cummings was playing a commanding role in responding to the Covid-19 outbreak. Cummings is understood to be close to Warner, whose brother, Marc, runs Faculty, an artificial intelligence company that the Guardian revealed is involved in an “unprecedented” data-mining operation as part of the government’s response to the coronavirus outbreak.

An accomplished data scientist, Ben Warner previously worked at his brother’s AI company, which has teamed up with Palantir, the US data firm founded by the rightwing billionaire Peter Thiel, to consolidate UK government databases to help ministers respond to the pandemic.