LABOUR launched a formal complaint with the Parliamentary Commissioner for Standards today to demand an investigation into how Boris Johnson’s holiday to Mustique was funded.

The PM and his girlfriend Carrie Symonds went on a post-election jaunt to the Caribbean island and stayed at a £15,000 luxury pad arranged by Carphone Warehouse co-founder David Ross.

Mr Johnson’s entry in the Commons Register of Members’ Interests recorded that the trip was a “benefit in kind” from Mr Ross.

However, Labour has said that while Mr Ross admits that he “facilitated the accommodation” other questions have been left unanswered.

“Boris Johnson must come clean about who has paid for his luxury trip,” said shadow Cabinet Office minister Jon Trickett.

“If he fails to do so, the Parliamentary Commissioner for Standards should step in and make him fess up.”

In a letter to the standards watchdog, Mr Trickett said: “The code of conduct requires members to provide the name of the person or organisation that actually funded a donation.

“Transparency is crucial to ensuring that the public have confidence that elected members of this house have not been unduly influenced by any donations or gifts that they may receive.”