The Institute for Fiscal Studies said George Osborne must instruct the Office for Budget Responsibility to discount the sum when assessing whether the Treasury is on track to meet its self-imposed targets.
The OBR, the Government’s independent fiscal and economic forecaster, will update its assessments to coincide with the Chancellor’s Autumn Statement on December 5.
The IFS said that because the QE changes – which will see the interest paid on gilts bought through QE transferred from the Bank of England to the Treasury – did not alter the long-term outlook for the public finances, the sum should be excluded.
“It would be inappropriate for [the Chancellor] to use these adjusted numbers as the sole basis for judging compliance against his fiscal rules.
“This would risk undermining the credibility with which he is managing the UK public finances.”
The IFS also warned that Britain’s austerity drive could last until 2018 because of the continued sluggishness of the economy.
“Since the budget the outlook for the UK economy has deteriorated and government receipts have disappointed by even more than this year’s weak growth would normally suggest,” said Carl Emmerson, deputy director of the IFS.
“In that case the planned era of austerity could run for eight years, from 2010-11 to 2017-2018.”
The think-tank said that VAT may have to be put up to 25pc to help plug the hole in the public finances.
The IFS added added that a weaker growth outlook and lower than expected tax revenues meant Mr Osborne would probably have to abandon his target to see debt as a percentage of GDP falling by 2015-16.
It warned that he may also have to announce further spending cuts and tax rises to meet his main fiscal target of balancing the structural current budget by the end of a five-year rolling period. That would potentially extend austerity to eight years from the five originally intended after the 2010 general election.
Increased pressure on the public finances puts Britain’s treasured AAA credit rating at risk. Moody’s said earlier this month that it would look again at the rating early on in the new year, after the Autumn Statement forecasts.
In better news for the Chancellor, the Office for National Statistics is expected to confirm its initial estimate that the economy grew by 1pc in the third quarter, after receiving more data on the period.
The CBI also reported a rise in business confidence in Britain’s services sector, while Experian said business insolvencies were 8.7pc lower in October, compared with a year earlier.