The rate-setters also decided to keep interest rates unchanged at their historic low of 0.5% for a fifth month.
BBC economics editor Stephanie Flanders said that the Monetary Policy Committee's (MPC) £50bn expansion of the programme had surprised many in the markets.
"Most expected the MPC to either put the quantitative easing policy on hold or simply spend the remaining £25bn authorised by the chancellor in March," she said.
"I think they believe that the fragility of the economy right now is such that there's more risk in doing too little than in doing too much," she added.
Following the announcement, the pound gave up much of its recent gains against the US dollar