A crackdown on Britons who hide money in Swiss bank accounts has recouped just £440 million so far this year despite George Osborne banking on it to boost Treasury coffers by £3.12 billion.
The Chancellor factored the cash into the Government's estimated income when he made his autumn statement last year but HM Revenue and Customs has collected "significantly less" than expected, MPs were told.
Edward Troup, tax assurance commissioner, suggested more would be recouped next year but admitted "it looks as though we are going to get less this year" than had been projected.
"We have received from the Swiss that amount (£440 million) and it is significantly less than the amount which we expected this year," he told the Public Accounts Committee (PAC).
The money is being collected under a treaty with Switzerland that came into effect this year that means banks impose a ''withholding tax'' on UK resident client assets and then transfer that money to HMRC.
PAC chairwoman Margaret Hodge said the target, set for the 2013/14 financial year, was an "Alice in Wonderland figure" that had been "totally unrealistic".
"Who is being held to account for that figure? We time and time again sit here.. and that's a lot of money, over £2 billion being fed into the public expenditure figures, who is being held to account?"
Mr Troup replied: "It's money that we would not have got through any other means. Second, it was the best estimate and I am happy to stand by it as the best estimate at the time of the money we expected to get this year."
HMRC analysts worked with the independent Office for Budget responsibility to come up with the £3.12 billion prediction, which was " our best estimate", he told the committee.
Tax officials had worked out that £40 billion was stashed in the accounts by "extrapolating" figures but did not know if that had substantially reduced following the publicity about the tax crackdown, he added.
Mr Troup admitted that the figures on how much the Treasury should expect to recoup - £ 5 billion over the six years - would have to be changed when the Chancellor gives his December mini-budget.
"I think we will revise the forecasts for the receipts in the next autumn statement," he told MPs.
Mr Troup said he had held discussions with his Swiss counterparts that had "left them in no doubt" about British concerns over the lack of money flowing back under the agreement but was not reassured by their response.
"They have taken that away," he told the PAC. "Did I come away completely reassured that we were going to get the full amount of the £3 billion? No, I did not."
So far, 18,000 names have been given to the HMRC by the Swiss authorities and letters have been sent to 9,000 of those, he told MPs.
But, HMRC has no "hard information" about the true amount of money held in the accounts.
"We still do not have sufficient information, we will would like more information and this is an agreement which has had to take account of Swiss banking secrecy," he added.
Shadow Treasury minister Shabana Mahmood said: "We warned two years ago that this deal had more holes than Swiss cheese. And now we learn that half way through the year the Government has raised just a fraction of the £3.1 billion George Osborne promised.
"If all this money does not materialise it will be a further blow to a deficit reduction plan which has already stalled because our economy flatlined for three years.
"After the decision not to close down the eurobonds tax loophole, this is yet more evidence that David Cameron's Government is totally failing to tackle tax avoidance. At a time when families face a cost of living crisis and the deficit is high, this simply isn't good enough."
Mr Troup was also pressed on HMRC's work to tackle the tax gap and conceded that while it had fallen as a percentage from 8.3% to 7% during 2011/12 it had gone up in cash terms.
He said: "I don't think we can take credit or take the blame for changes in the overall economy or policy decisions which the government makes to cut or increase tax rates but those are all reflected in tax receipts."
" If it was still 8.3 now we would be losing £7 billion more tax than we actually are through the tax gap," he added.
Mrs Hodge said that in 2011/12 HMRC collected an extra £1 billion in tax overall, an additional 0.2% from what had been collected the previous year.
"It's a cash sum so in real terms we got less in 11/12 than we did in 10/11," she said.
Mr Troup said HMRC had no projections for how much could be collected if major companies that have faced accusations of tax dodging were taken into account.
"It's a policy judgment," he told the PAC.
Mrs Hodge said: "The tax gap is really the tip of the iceberg between the money that you collect and the money that you would really be owed if everybody paid their fair share according to either their individual wealth or according to the profits they made from economic activity."
The PAC also pressed Mr Troup on claims he had once suggested that "taxation is legalised extortion. He admitted "those words appeared" in an article he once wrote an article in the Financial Times in the 1990s.
"I do have to admit, I did write those words," he added.