Ministers have been accused of a "shocking" lack of transparency over their chauffeur-driven car use amid signs that costs have risen.
The Government has failed to produce supposedly "annual" figures for spending on official cars for three years since a new system was introduced.
The bill was said to have fallen to £1.9 million in 2011-12 - but a response to a parliamentary question last week stated that the revamped Government Car Service (GCS) cost more than £6.3 million last year.
The Department for Transport (DfT) suggested the sums were not directly comparable because its 2011-12 figures had not reflected the full cost to the taxpayer.
It appears there may now be no way of knowing how ministers are using official vehicles, as a number of Whitehall departments are not recording "individual journeys" and who is making them.
The Press Association filed a Freedom of Information request with the Department for Work and Pensions (DWP) asking for details of car trips made by Baroness Mone, the Ultimo underwear firm founder who has been appointed David Cameron's entrepreneurship tsar.
Lady Mone came under fire in September after posting a photograph of a Jaguar steering wheel and dashboard with the message: "The things you do ... in Government car drying my travel top ... love it so much."
The peer was said to have been taken from her London home to Euston station while working on a review for DWP. But responding to the FOI, the department said it had not kept any records of her journeys.
"Since 1 October 2012, a departmental official car has been available for official journeys undertaken by ministers," the response said.
"When not required by ministers it may be used by those conducting official departmental business where it is cost-effective to do so.
"A daily mileage record is kept but details of individual journeys are not held."
Answering a parliamentary question about official cars in September last year, Treasury minister Andrea Leadsom said her department "does not keep a record of ministers' individual car journeys".
The coalition set about reforming official car use in 2010, slashing the number allocated to specific ministers and introducing a central ministerial car "pool" that could be accessed when the need arose. But this arrangement was quickly scrapped after officials complained it was "unsustainable".
The Government Car and Despatch Service (GCDS) agency was abolished at the end of September 2012, and the GCS was brought in-house at the DfT. Alongside allocated cars for senior ministers, GCS now provides departments with their own pool vehicles that can also be used by officials.
The DfT said car use costs for 2012-13, 2013-14 and 2014-15 were due to be published in a written statement to Parliament "soon". A spokesman said its £1.9 million figure for 2011-12 had only represented the charges paid by departments, rather than the real costs to the taxpayer of providing cars for ministers.
As a result it could not be "compared as like for like" with the £6.325 million quoted by transport minister Robert Goodwill for 2014-15, which covered "all running costs, ie rent and rates, driver and other staff salaries".
The spokesman claimed the equivalent bill for the service in 2010-11 had been around £21 million - although that included the costs of the now-defunct Government Mail Service and does not appear to tally with figures in the GCDA accounts.
The spokesman said: "We are committed to ensuring that the GCS provides value for money for the taxpayer.
"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 and senior officials. After April 2012, cars were allocated to departments as departmental pool cars for use by ministers and senior officials. These cars are managed by each department."
Labour MP Fabian Hamilton branded the situation "shambolic" and said he was "appalled" by the failure to produce data that would allow ministers to be held to account.
He said it seemed "bizarre" that the DfT was now suggesting the figures released in 2011-12 had been an underestimate. He also questioned whether there could be a link with embarrassing criticism of Sir Simon Burns in January 2013 - when parliamentary questions revealed the then-rail minister had been using an official car to commute from his Chelmsford home to Westminster.
"I am a bit shocked because I cannot see why they would have stopped recording journeys," Mr Hamilton said. "It does seem surprising that at about the time Simon Burns was exposed the individual recording of journeys stopped.
"It seems to me at a time when government departments are being slashed to the bone and MPs and government servants are - rightly - being asked to account for every expense, the Government Car Service is not accounting for what it does.
"How do you even know if there is a cost saving if there is no adequate collection of data?"