The government is facing “significant questions” over the credibility of its coronavirus decision-making after it emerged that Boris Johnson‘s chief adviser Dominic Cummings participated in meetings of its top scientific experts.

Opposition MPs reacted with fury to reports that Mr Cummings and Ben Warner, another No 10 adviser, attended a crunch meeting of the Scientific Advisory Group for Emergencies (Sage) on 23 March, the day before the prime minister announced the lockdown.

Membership of Sage is a closely guarded secret but ministers have faced increasing pressure to disclose who sits on the panel and the advice it has given the government on tackling the outbreak.

A leaked cast list, seen by The Guardian, said that Mr Cummings and Mr Warner, a data scientist who worked with Mr Cummings on the Vote Leave campaign, have both attended meetings.

Downing Street insisted the pair were not ”members of Sage” and had attended some of the group’s meetings “to understand better the scientific debates concerning this emergency” and “understand better the limits of how science and data can help government decisions”.

In a strongly worded statement, No 10 claimed “public confidence in the media has collapsed during this emergency partly because of ludicrous stories such as this”.

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But Sage participants told the newspaper that Mr Cummings and Mr Warner had been actively participating in discussions rather than observing, raising questions about the impartiality of the advice given to ministers.

Sir David King, a former government scientific adviser, said he was “shocked” to discover political advisers were involved in the meetings.

“If you are giving science advice, your advice should be free of any political bias. That is just so critically important,” he told The Guardian.

Sir David later tweeted: “I don’t say this lightly but if this report is accurate (which I have no reason to doubt) it marries with all of my worst fears. This is simply unimaginable, an egregious abuse SAGE membership the govt must answer.”

Shadow health secretary Jonathan Ashworth said: “These are very serious revelations and raise significant questions about the credibility of decisions being taken by the government on the coronavirus.

“Dominic Cummings has no place on the government’s scientific advisory group on the coronavirus. He is a political adviser, not a medical or scientific expert.

“If the public are to have confidence in the Sage, the government must make clear Dominic Cummings can no longer participate or attend.

“We also need full transparency on who is attending meetings of Sage and what is being discussed.”

Sir Ed Davey, the acting Liberal Democrat leader, said the revelations underlined the need for an independent inquiry into the handling of the coronavirus outbreak.

“The public needs to have confidence that it is expert advice that is guiding government decisions, not dubious political advisers,” he said.

“The lack of transparency is an absolute outrage.”

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A government spokesperson initially said only: “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.”

But in a further statement issued late on Friday, Downing Street added: “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.

“They do this in order to understand better the scientific debates concerning this emergency and also to understand better the limits of how science and data can help government decisions.

“Occasionally they ask questions or offer help when scientists mention problems in Whitehall. Others also listen to meetings without being ‘on’ or ‘a member of’ Sage.

“Sage provides independent scientific advice to the government. Political advisers have no role in this.

“The scientists on Sage are among the most eminent in their fields. It is factually wrong and damaging to sensible public debate to imply their advice is affected by government advisers listening to discussions.”

It comes after England’s chief medical officer Professor Chris Whitty said neither he or Sir Patrick Vallance, the chief scientific adviser, opposed the membership of Sage being revealed.

He told MPs: ”Neither of us have any problem in principle with the names being made public, many people talk about their own work on Sage perfectly legitimately.