UPDATE 1-UK PM will not soften austerity despite IMF growth cut

(Adds quotes from Sky interview, context, details)

BIRMINGHAM, England, Oct 9 (Reuters) - British Prime Minister David Cameron said on Tuesday he would not soften his austerity programme with a "Plan B" of slower spending cuts after the International Monetary Fund downgraded its growth forecasts for Britain.

The IMF cut its UK economic growth estimates on Monday, predicting the economy would shrink 0.4 percent this year before growing by 1.1 percent in 2013. In July, the IMF had said Britain's economy would grow 0.2 percent this year and 1.4 percent in 2013.

The IMF also said Britain should delay some spending cuts pencilled in for next year if growth turned out to be much weaker than forecast, but Cameron said that did not mean his Conservative-led government should change its plans yet.

"What we need in Britain is not 'Plan B', which is more borrowing. How can you borrow your way out of a debt crisis?" Cameron told Sky News.

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

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

(Reporting by Matt Falloon; Editing by Alessandra Rizzo)