Chauffeuring ministers' red boxes has cost £350,000
Chauffeuring ministers' paperwork around Whitehall has cost more than £350,000 since 2011, figures show.
A Freedom of Information request by the Huffington Post UK found the cost of a journey transporting just government documents had often exceeded £100.
The total bill could be higher as figures for additional trips using pool cars were not available.
The government said there had been a "substantial reduction" in the cost of ministerial cars since 2011.
David Cameron announced plans to shrink the bill for ministerial cars shortly after coming to office in 2010, and the number of ministers with access to their own chauffeur-driven car was cut.
The practice of carrying ministers' "red boxes" in official cars is used for sensitive documentation.
In 2010, journalist Sarah Vine, the wife of the then education secretary Michael Gove, wrote that while her husband could travel home by public transport, his red box would travel separately "in air-conditioned splendour, snug and secure in the back of a locked car".
The red boxes travelled alone using the Government Car Service 1,910 times between 2012-13 and 2014-15, with the number of journeys rising over the course of the three years.
Cost of carrying ministers' paperwork
Source: Freedom of Information request to the Department for Transport
Since 2012, pool cars have also been used by departments but their use is not recorded by the DfT, the department said.
Labour MP Graham Jones told the Huffington Post: "The Tories said they'd clamp down on wasteful spending yet they still allow hundreds of thousands of pounds of taxpayers' money to be spent ferrying round briefcases in chauffeur-driven cars."
But the Department for Transport said the total cost of carrying ministers' paperwork was just 5% of the total spent on the Government Car Service and that it was committed to ensuring the service provided value for money.
A spokeswoman added: "That is why, since 2010, the number of cars that make up the service has been reduced from 227 to 78.
"We have also been working hard to cut our operating costs in order to further reduce the cost to the public purse.
"Before April 2012, all cars were part of a government car pool and could be booked separately by ministers. After April 2012, cars were allocated to departments as departmental pool cars for use by ministers. These cars are managed by each department."