David Davis is reportedly facing an investigation by the Cabinet Office after spending thousands of pounds travelling back and forth to Europe.
The Independent claims to have seen a letter from the Cabinet Secretary, Sir Jeremy Heywood, asking whether Mr Davis's trips break the ministerial code's rules on "cost-effectiveness".
Labour MP Stephen Doughty called for an investigation into whether the flights broke the ministerial code.
He said: "After seven years of austerity, the British people rightly expect those who govern our country to do everything they can to minimise the cost of their transport to the public purse wherever possible – not least when huge cuts are being threatened to our armed forces."
The data reportedly reveals the Brexit Secretary used RAF planes during eleven trips between December 2016 and September 2017.
A spokesman from the Department for Exiting the European Union told The Independent the Secretary of State often uses other means of travel when appropriate, such as the Eurostar, adding that he has also taken a number of commercial flights.
The spokesman said: “The Brexit negotiations are critical to the nation's future and it is absolutely right that the Secretary of State has access to whatever mode of transport is most appropriate.
“RAF aircraft have been used to transport senior ministers from Governments over many years because they provide security, flexibility and value for money, especially when travelling in large groups and in multi-legged trips.”
