LONDON (Reuters) - British Prime Minister David Cameron said on Tuesday he would not soften his austerity program with a "Plan B" after the International Monetary Fund downgraded its growth forecasts for Britain.

"These are difficult times for the economy and what the IMF report is doing is coming into line with other forecasters who have already said that growth is going to disappoint right across Europe this year. We know that," Cameron told BBC.

"The IMF also say we shouldn't abandon our plans in making reductions in government spending and also regrettably in some cases put up some taxes to get on top of our debt and our deficit."

"It's not 'Plan B' that we need. What we're doing is making sure that every part of 'Plan A' is firing on all cylinders," he added.

The IMF said Britain might have to defer some of its spending cuts if growth turns out much worse than expected.

(Reporting by Matt Falloon; Editing by Alessandra Rizzo)