Deputy Leader of the Opposition Angela Rayner has criticised Rishi Sunak after the Chancellor received a 'partygate' questionnaire from the Metropolitan Police.
Mr Sunak was present during Boris Johnson's birthday party in the Cabinet Room on 19 June 2020.
He has claimed to have been there awaiting a Covid-19 meeting.
The MP for Richmond in North Yorkshire initially told media he did not expect to receive a questionnaire.
But, on Thursday 25 February, a Treasury source confirmed the Chancellor is among the more than 50 people to have done so under Operation Hillman, which is looking into 12 reported gatherings.
Speaking to ITV Tyne Tees, Labour's Ms Rayner called the scandal a "national embarrassment", saying it undermines democracy and political messaging.
"It’s a national embarrassment that we’ve got senior members of the cabinet involved in shenanigans which is rule breaking and lawbreaking and they’ve not been able to come clean about it," she said.
"We’ve got very serious issues that are happening now and when you break your own rules you undermine the very democracy democracy and the messages we are trying to deliver to people."
Earlier this week, ITV News obtained a copy of one of the questionnaires that have been sent out.
The Treasury has said it was an 'inevitability' due to the Chancellor living and working in Downing Street.
After ITV News first reported the allegations in January, a spokesperson for Mr Sunak said: "The Chancellor went round to the cabinet room for a Covid strategy committee meeting, which was being held in the same room."
People in Richmond give their verdict on the investigation into Sunak
Last week ITV News revealed that those under police investigation would be allowed to view the responses they gave in interviews with civil servant Sue Gray before they respond to the police questionnaire.
Ms Gray - who conducted the original internal investigation before passing police evidence which pushed them into announcing their own criminal probe - said people would only be able to view their own answers and no one else's.
Among the package of evidence passed to police by Ms Gray were 300 photographs relevant to the partygate investigation - one of which allegedly shows the PM holding a beer.
The Met has confirmed that in line with policy it would not reveal the people who are issued with FPNs, however Number 10 has said it would be made public if the prime minister is fined.
Fines start at £100 for the first offence, growing to £200 for the second offence before doubling for each repeat offence before hitting the cap of £6,400.
Any individual is entitled to appeal their fine, which could see the appellant appear in court.