Chancellor Gordon Brown is "past his sell-by date" and has "held Britain back", the shadow chancellor George Osborne has told MPs.
Mr Osborne, an ally of Tory leadership favourite David Cameron, called the chancellor a "roadblock to reform".
In his pre-Budget report, Mr Brown halved his pre-election estimate for UK economic growth in 2005 to 1.75%.
Mr Osborne said what Mr Brown "told the country at the time of the last election was not true".
During his speech, Mr Brown blamed oil prices, a slowing housing market and inflationary pressures for a lower economic growth forecast than he had predicted in March.
But Mr Osborne said the economy of the UK, an oil exporter, was growing more slowly than those of "other oil-importing competitors".
He argued that Mr Brown was "simply not rising to the challenge".
Mr Osborne said: "I don't remember him telling the electorate before the General Election that the economy was facing a tough and challenging year.
"What he told the country at the time of the last election was not true."
'Not brutal enough'
He ridiculed the speech as sounding "more like the tractor production figures from the old Soviet Union".
He said it had been a "tragic story" of a chancellor who had been forced to wait so long for Number 10 his reputation "is crumbling".
"His golden rule is now tarnished and discredited, productivity growth has slumped, business investment has collapsed and this year Britain has some of the weakest economic growth in the developed world," Mr Osborne told MPs.
But Mr Brown responded to Mr Osborne by saying his rival had accused him of being "too brutal with him over the last few months".
He added: "I have to say, after having heard his speech, I've not been brutal enough.
"They say the Conservatives are about to skip one generation. Perhaps it is time they skip another one as well."