The number of people out of work and claiming unemployment benefit is set to go back above a million in 2007, Gordon Brown's pre-Budget report suggests.
Unemployment has been rising steadily
Lib Dem treasury spokesman Vincent Cable highlighted the figure, in the "key assumptions" contained in the small print of the document.
He asked Mr Brown to confirm "an alarming" prediction - nearly six years after the total fell below a million.
Mr Brown said the government did not forecast future unemployment rates.
Mr Cable held aloft the pre-Budget red book, showing Mr Brown page 224 and asked Mr Brown to confirm the figures.
The tables on that page appear to show the assumption that the number of people claiming unemployment benefit will rise from 961,000 to 1,010,000 million in 2007-08.
Mr Brown replied: "I have to tell him that it's not been the history of any government to project what unemployment is going to be a year from now."
The claimant count had fallen from 1.6 million in 1997 when Labour had come to power, he added.
Mr Cable later said: "This is the first acknowledgement by the Treasury that unemployment will continue to rise, despite the optimistic forecast for the economy.
"My particular anxiety is that rising unemployment is contributing to rising home repossessions, because very few home buyers are properly insured against loss of income, and there is little support from the benefits system."
But a Treasury spokesman said: "As stated clearly in the pre-Budget report, it is not the Treasury's assumption that the unemployment claimant count is rising above one million, the figure is based on the average of external forecasts. It is misleading to suggest otherwise."
There are two main ways of measuring unemployment.
The claimant count referred to in this case was the official one during the 1970s, 1980s and 1990s when unemployment was a key electoral issue.
Its fall below the symbolic one million mark in February 2001 was the first time it had done so for more than two decades.
At the time, Prime Minister Tony Blair said that "full employment is within our grasp for the first time in a generation".
However, another measure to count unemployment - the ILO labour force survey - is at 1.71 million.
When Labour came to power in 1997, it said it preferred the ILO to the claimant count.
In the Commons, Mr Brown dismissed as "completely untrue" opposition claims that Britain has the highest rate of unemployed young men in the developed world.
He told the Tories that instead of "attacking the unemployed they should be attacking unemployment".
Shadow finance minister Paul Goodman asked Mr Brown to confirm the "simple but little known claim that unemployment among young men in Britain is now the highest in the whole developed world".
The chancellor said: "It is completely untrue."