Mr Johnson, his wife Carrie and their one-year-old son Wilfred stayed in a villa owned by Conservative peer and friend of Mrs Johnson Zac Goldsmith, and was provided free of charge, according to the latest register of Ministerial Interests.
However, this trip does not appear in the Common’s register for Member’s interests.
A spokesman for Boris Johnson has said that all transparency requirements laid down by the ministerial code and the code of conduct for MP’s had been met when it came to Mr Johnson’s trip to Marbella after Conservative party conference last month.
The spokesman explained: “Earlier this year, the Prime Minister received hospitality from a longstanding friend who provided use of their holiday home.
“The Prime Minister’s met the transparency requirements in relation to this, he declared this arrangement in his ministerial capacity, given this was hospitality provided by another minister.”
He said the PM’s ministerial standards adviser Lord Geidt had scrutinised the declaration as part of the process and the entry to the ministerial register notes Mr Johnson has a “longstanding personal friendship with the Goldsmith family”
Number 10 would also not be drawn on how much the five-day-long trip had cost.
Labour have asked the Standards Commissioner to open an investigation, with Deputy Leader Angela Rayner saying that the omission “appears to be a breach of the House Code of Conduct.”
Ms Rayner said in a letter to commissioner Kathryn Stone that Mr Johnson “has a long history of breaching the rules in relation to parliamentary standards and other integrity and anti-corruption measures”.
The letter added: “We cannot have a situation where Boris Johnson behaves like it’s one rule for him and another for everyone else.”