Transport Secretary Chris Grayling has come under fire for being in Qatar as rail fares increased.
Labour accused him of refusing to defend his record by making the visit on the day passengers were being hit by an average hike in fares of 3.4%.
Mr Grayling went to Qatar to meet members of the Gulf state's government, including the prime minister, as well as chief executives of the Qatar Investment Authority and Qatar Airways.
Asked about the purpose of Mr Grayling's two-day visit, which will be followed by a day in Turkey later this week, Prime Minister Theresa May's official spokesman told reporters: "There are ministers visiting a whole host of countries, spreading the message that Britain is a very good place to invest and to do business in.
"Chris Grayling obviously plays an important part in that."
Mr Grayling has been named as a possible candidate for the axe in a forthcoming reshuffle widely predicted in the press over the Christmas period.
Asked if he would remain in post for the foreseeable future, the PM's spokesman responded: "Chris Grayling is working hard and doing a good job as Transport Secretary."
Shadow transport secretary Andy McDonald said: "The Secretary of State for Transport's failure to publicly explain to rail passengers why they are being hit with crushing fare increases today smacks of a man running scared.
"Chris Grayling won't defend his multimillion-pound bailout of Stagecoach on the East Coast line because he can't.
"Passengers and taxpayers deserve better than a failing Transport Secretary who refuses to defend his track record."
Liberal Democrat leader Sir Vince Cable said: "Rail passengers are shivering on platforms, angered by the biggest fare increase in years, while Chris Grayling is off globetrotting.
"It's very difficult to see what useful function he can perform in Qatar and Turkey that our excellent trade officials could not."
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "Chris Grayling knew that the fares story would be top of the news agenda today but, instead of being available to defend his Government's great rail rip-off, he booked himself a trip to the Qatari sunshine.
"While millions of passengers are taking a financial hit as they battle their way back to work in the cold and the rain today, they will draw their own conclusions from the Transport Secretary's decision to book himself a trip to the desert."