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.”

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.