Government data suggesting nearly 40% of motorcycles are untaxed could have been made up, an MP has said.
The suggestion came as senior civil servants responsible for the data were hauled over the coals by the Commons public accounts committee.
Committee member Angela Browning MP said researchers contracted to note down number plates could have made them up instead, increasing the rate found to have no valid tax record.
She highlighted the fact contractors were told to record a minimum number but given no targets for accuracy.
Addressing Anthony Boucher, head of statistics travel division for the DfT, she suggested a contract should have "required the people taking the numbers manually to at least have a certain percentage rate of accuracy attached to them. Otherwise people could have just sat there and made up anything," she added.
Asked by Browning why there was no accuracy target, Boucher sheepishly replied: "I don't know."
A new Government roadside survey published in February - and using cameras to record number plates - put the rate of untaxed motorcycles at just 6.5%.
Boucher was called to account alongside Dr Stephen Hickey, DfT’s director general of safety, service delivery and logistics.
Committee chairman Edward Leigh MP said the committee had suffered "enormous embarrassment" after publishing a report based on the earlier figure in January.
When the committee published its report, Leigh had said in an accompanying statement: “Motorcyclists are particularly liable to evade road tax. Nearly 40 per cent of motorcycles are now unlicensed… Large parts of the biking community are cocking a snook at the law.” He was forced to apologise to motorcyclists when the new figure emerged.
Furious Leigh told Hickey: "In the seven years that I've been chairman of this committee, Dr Hickey, I think this is one of the most serious issues that we've had to deal with."
Leigh attacked Hickey for not listening to the motorcyclists. "You were warned by the motorcycle community that something must be wrong there. Why didn't you come clean with this and say that clearly there's something wrong there, we can't place any reliance on these figures?" he asked.
Hickey apologised for embarrassment suffered by the committee but claimed: "We didn't know until we ourselves saw the results, which obviously was just before they were published."
Leigh responded: "I don't, frankly, believe that."
Watch Hickey and Boucher facing the music here: http://www.parliamentlive.tv/Main/VideoPlayer.aspx?meetingId=1576&rel=ok