Telegraph:

The Department for Business officials thought the £400m scrappage scheme , which ends next week and provides a £1,000 subsidy that is matched by manufacturers, could create a short term boost of £349m from additional tax receipts to the UK but a long term loss of £55m. They revised those estimates down last autumn after the scheme was extended to benefits of £116m and losses of £18m.
The National Audit Office discovered that officials made clear this was not value for money, but Lord Mandelson circumvented the department’s accounting officer and pushed ahead.
“The minister decided to proceed with the scheme for several reasons including that sales made during a recession were worth more than those made in the future, when the sector had recovered, and the risk of doing nothing outweighed the possible scheme costs,” the report stated.
It also estimated that more than half the additional new car sales since the scheme’s launch last May would have happened anyway in the next few years without cost to the public purse.