Blair warns on UK referendum on EU membership

Britain's membership of the European Union hangs on the outcome of a knife-edge election in four weeks' time, as former British leader Tony Blair weighs into the debate.

Former British prime minister Tony Blair. (AAP)

British Prime Minister David Cameron's plan to hold a European Union membership referendum if he is re-elected next month would lead to an intense period of 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, and then hold a referendum by the end of 2017.

In a campaign speech on Tuesday, former Labour leader Blair, prime minister from 1997-2007, said the "short-term" pain of Cameron's proposed re-negotiation has been underestimated.

"Think of the chaos produced by the possibility, never mind the reality, of Britain actually quitting Europe," Blair, 61, said.

"Jobs that are secure suddenly insecure; investment decisions postponed or cancelled; a pall of unpredictability hanging over the British economy."

If Britain voted to leave the EU, business would face the most intense uncertainty since World War Two, Blair said.