British Prime Minister David Cameron's plan to hold a European Union membership referendum if he is re-elected next month would lead to intense uncertainty for businesses, former British Prime Minister Tony Blair said on Tuesday.

Cameron, whose Conservatives are neck-and-neck with the opposition Labour Party in most polls ahead of the May 7 vote, has promised to renegotiate Britain's ties with Brussels ahead of a referendum by the end of 2017.

In a campaign speech in his former constituency in Sedgefield on Tuesday, Blair, who served as prime minister from 1997-2007, said that the "short-term" pain of Cameron's proposed renegotiation has been underestimated.