Mohamed El-Erian, chief executive of Pimco, warned that David Cameron would need a ‘Plan B’ to limit the impact on the economy if the UK were to exit the EU.

“Whichever way you look at this, Prime Minister David Cameron has materially increased the probability that, beyond 2017, Poland rather than the UK will be among the three largest economies defining the scale and scope of European regional integration.

“Let’s hope that he also has a Plan B that would limit the potential downside to Britain’s standard of living,” Mr El-Erian wrote in Fortune magazine.

The warning followed the pledge by Mr Cameron last week to hold a referendum on Britain’s membership of the EU in 2017 should the Conservatives win the next election.

Mr El-Erian said that while the promise was likely to “score some internal political points” it risked “some reduction in the country’s growth”.

“At the minimum, it [the referendum] complicates ongoing efforts to overcome the region’s debt crisis. It could also detract from the eurozone’s multi-year recovery process, structural elements of which need to be put in place rapidly,” he said.