Boris Johnson's former spokeswoman quit earlier this month after a clip emerged of a mock press conference held between Downing Street aides and Ms Stratton. In the video, they laughed about a "fictional" lockdown party just days after an alleged event took place.

       The Mail on Sunday reports that former and current No 10 staff fear more footage could come out.

       A source told the newspaper "the worst could be yet to come" on rehearsals for the subsequently ditched televised press briefings.

       The source said: "Allegra is of course well aware she was asked more than just the one question we have all seen.

       "She is feeling very anxious and has totally gone to ground, she is speaking to hardly anyone.

       "She and her former colleagues in Downing Street have convinced themselves they know the source of the leak.

       "They believe that a confidentiality agreement has been breached.

       "They are living in fear every day that the rest of the questions put to her will come out, because they know they were filmed.

       "Any further breach would be in the public interest, so they feel they cannot stop it.

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       "They are just waiting like sitting ducks. To say they are on tenterhooks is an understatement."

       The source added: "The worst could be yet to come. Other mock scenarios included fictional questions about the mistresses of Boris and his love children.

       "No 10 will need to brace themselves. They are absolutely s***ing themselves about what else is around – and Allegra is too. She knows what was asked and how she answered.

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       "That was not a failing on the part of the teams involved in the process – they would not be doing their jobs properly if they hadn't worked out what the most difficult questions were likely to be and prepared for them.

       "But it just goes to show what a ridiculous, self-defeating plan it was from the start."

       It comes at a turbulent time for the Prime Minister as he faces potentially the most damaging week in his premiership.

       Allegations of lockdown-breaking parties during coronavirus restrictions last year have sparked fury.

       And Mr Johnson suffered his largest rebellion yet earlier this week as nearly 100 Tory backbenchers voted against Covid passes.

       The Conservatives were also dealt a humiliating defeat in the North Shropshire by-election on Friday.

       In the latest body blow to the Prime Minister, Brexit minister Lord Frost has resigned citing "concerns about the current direction of travel" of the Government.

       Express.co.uk has contacted No10 for comment.