The prime minister’s official jet was used for a “boozy jolly” by civil servants, according to a report.

Sky News understands a number of officials joined the 91-minute journey over the UK – and that it took place with “usual catering for a flight”.

The Sun newspaper claims that during the 700-mile trip – which reportedly cost £50,000 – a “fancy meal with a selection of alcoholic drinks” was served.

Flight data shows it took off from Stansted and headed to the Lake District before returning.

A statement on behalf of the government, given to Sky News, said: “In order to comply with Airbus and aviation industry rules, the aircraft was legally bound to operate a maintenance flight before 4 September or face significant additional storage costs.”

It is claimed that the trip by a number of Foreign Office civil servants was necessary to ensure that – following “a recent reconfiguration of the aircraft” – it would “still meet ministerial requirements”.

Labour’s shadow trade secretary Emily Thornberry told The Sun: “This would be utterly disgraceful behaviour at the best of times, but in the middle of our country’s current crisis, it is shameful beyond words.”

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The RAF Voyager plane is shared between Boris Johnson and the Royal Family. Once grey, it was resprayed white, and given a Union Jack on its tail in a controversial £900,000 makeover.

The news of the alleged “jolly” comes as Boris Johnson and his allies have launched a legal fightback against a Commons inquiry into claims he lied to parliament about partygate.

The inquiry into Mr Johnson, by the all-party Privileges Committee, has been denounced as “a witch-hunt” and “a kangaroo court” by his allies after the committee announced it could rule against him even if he did not deliberately mislead MPs.