UPDATE 2-UK's Cameron won't ease austerity after IMF downgrade

* Cameron says IMF just catching up with other forecasters

* Britain behind on austerity targets due to weak economy

* Economists expect UK deficit plan to be extended in Dec

(Adds details, Labour quote)

By Matt Falloon

BIRMINGHAM, England, Oct 9 (Reuters) - British PrimeMinister David Cameron said on Tuesday he would not soften hisausterity programme with a "Plan B" of slower spending cutsafter the International Monetary Fund downgraded its growthforecasts for Britain.

The Conservative-led government, steadfast in its ambitionto cut a record budget deficit but under pressure to fix arecession-hit economy, abandoned its original deficit reductiontargets last year and could be forced to extend them again.

"What we need in Britain is not 'Plan B', which is moreborrowing. How can you borrow your way out of a debt crisis?"Cameron told Sky News, speaking from the Conservative Party'sannual conference in the central English city of Birmingham.

"What we need is what I call 'Plan A+', we need to keep ourplans, difficult though they are to cut public spending and dealwith the deficit, but we need to add to that every measure thatbusiness has been asking for. And that's what we're doing."

The IMF cut its UK economic growth estimates on Monday,predicting the economy would shrink 0.4 percent this year beforegrowing by 1.1 percent in 2013. The IMF forecast in July thatBritain's economy would grow 0.2 percent this year and 1.4percent in 2013.

The IMF also said Britain should delay some spending cutspencilled in for next year if growth turned out to be weakerthan forecast, but Cameron said that did not mean there was anylogic in ripping up his plans yet.

"The IMF are not saying change course, they are saying stickto your plan unless things get dramatically worse," he said.

Britain's economy fell back into recession at the end of2011. Analysts expect Britain to return to growth in the thirdquarter but say that any recovery will be sluggish at best.

Critics, including the Labour opposition, blame governmentausterity measures for choking off demand but ministers saycutting more slowly would put Britain's market credibility atrisk at a time when the euro zone is embroiled in a debt crisis.

"These downgraded IMF forecasts are another damaging blow tothe government's economic credibility," said Labour financespokesman Ed Balls.

Finance minister George Osborne is expected to eitherannounce deeper cuts or to further extend his deficit reductionplan beyond the fiscal year 2016/17 when he delivers updatedeconomic growth and borrowing forecasts on Dec. 5.

Osborne had originally pledged to all but erase Britain'sdeficit by the time of the 2015 election.

(Editing by Louise Ireland and Alessandra Rizzo)