(Adds detail, quote from Prime Minister's spokesman)
By Christina Fincher and Matt Falloon
LONDON, Jan 28 (Reuters) - Britain must drain an extra 20 billion pounds from the economy by 2015 to get public finances back on track and it may take over 20 years to return public debt to pre-crisis levels, a leading think tank said on Wednesday.
The Institute of Fiscal Studies said, without that fiscal tightening, government borrowing could rise to more than 60 percent of gross domestic product, from where it would decline only gradually over subsequent decades.
Any further fiscal stimulus -- on top of the 20 billion pounds injected into the economy in November's prebudget report -- should not be set in train without consulting the Bank of England, the think tank said.
"Public support from the Bank of England for any further fiscal stimulus measures would help bolster credibility."
A spokesman for Prime Minister Gordon Brown said the report was not a condemnation of November's fiscal stimulus.
"The report makes clear that the cost of doing nothing would have been greater than the cost of the action we are taking," he told reporters.
There is growing speculation the government will introduce more economy-boosting measures at its annual budget this year, especially as figures this month showed the economy shrank at its fastest pace since 1980 in the three months to December.
RADICAL TIGHTENING
But the think tank noted there was a danger that even more radical fiscal tightening may be required if investors' appetite for British government bonds diminishes.
Markets have been rife with concern over Britain's economy, its financial system and the deterioration in the government's finances, sending the pound down to 23-year lows against the dollar and a record low against the yen.
"There is clearly a danger investors will take fright at the state of the UK public finances, pushing up gilt yields," the IFS report said.
"If borrowing costs were to return to the average levels of the 1990s, then further tax increases or spending cuts would probably be required to stop debt and debt interest costs rising unsustainably."
In November's pre-budget report, the government signalled spending cuts and tax increases starting in 2010/11 and raising 2.6 percent of national income by 2015/16.
The study calculates that if the public finances evolve as the government hopes, this tightening would have to remain in place until the early 2030s before debt returned to below 40 percent of national income -- the ceiling Brown set as one of his two fiscal rules in 1997 when he was finance minister.
"There is no prospect of a government being able to readopt these rules any time soon," the study said.