LONDON (Reuters) - The Bank of England stepped up its campaign to boost the struggling British economy, raising the size of its asset purchase programme on Thursday in a surprise move tantamount to printing an extra £50 billion ($76 billion) to get banks lending again.

The central bank kept rates at a record low of 0.5 percent as expected, but surprised markets by moving swiftly to reassure them that its scheme to buy up government and corporate bonds with newly-created money would last at least three months more.

Sterling lost ground against the euro and dollar on the news, but gilt futures more than halved earlier hefty losses that had taken bond yields back up to levels last seen in February, before BoE Governor Mervyn King had seriously mooted the policy of quantitative easing.