DETECTIVES have started interviewing "key witnesses" about alleged Downing Street parties during lockdown, reviving the scandal that had been threatening to topple Boris Johnson.

The Metropolitan Police confirmed it had sent out more than 100 questionnaires to people at gatherings in Number 10 and elsewhere in Whitehall during the pandemic. 

The Prime Minister and Chancellor Rishi Sunak were among those asked to complete and return the forms within seven days.

Investigators have started reviewing all available evidence but have not yet made any referrals for potential fines to be issued.

The development comes as Mr Johnson’s position had been looking more secure because of the war in Ukraine.

Tory MPs who had wanted him to quit over partygate, including Scottish Tory leader Douglas Ross, said responding to the conflict had to come first, with unity a priority.

Mr Johnson’s allies have also argued that it would be hugely unhelpful to the west’s response to Russia’s invasion for the UK to be distracted by a leadership crisis.

Brexit minister Jacob Rees-Mogg last week dismissed the partygate row as “fluff”.

In a statement, the Met said: “In addition to the detailed review of all available material, including returned questionnaires, detectives from the Operation Hillman investigation team have started interviewing key witnesses.

“This investigation involves a significant amount of investigative material; the serving of over 100 questionnaires and the need to individually assess every response.

“The offences under consideration comprise a number of elements and the legislation itself changed between the event dates. We are progressing the investigation as quickly as possible

“As yet, we have not made any referrals to the ACRO Criminal Records Office for the issuing of fixed penalty notices.

“However, every questionnaire response is being assessed alongside all available evidence, and should this reach the evidential threshold, then referrals will be made.”

The force is investigating 12 events, up to half of which may have been attended by Mr Johnson.

The latest questionnaire figures are double the number issued a month ago. 

The Met said: “As a result of responses so far, further individuals have been identified and questionnaires sent to them. As the investigation continues, we may need to contact more people as further information comes to light.”

Mr Johnson has faced repeated calls to quit over his attendance at some of the events.

Labour leader Sir Keir Starmer has changed tack recently, saying Ukraine comes first.

However Nicola Sturgeon and Liberal Democrat leader Sir Ed Davey have both said they still believe Mr Johnson is unfit for office and should go.

Once the Met investigation is complete, Mr Johnson has said MPs will also seee the full report into the affair compiled by Whitehall ethics veteran Sue Gray.