BORIS Johnson is expected to call for reform of the European Union in a speech on Wednesday and back a report by his economic adviser that says leaving the EU is a "viable option" for Britain if it cannot change, according to reports.

Prime Minister David Cameron has promised to renegotiate Britain's ­relationship with the EU ahead of a referendum on membership by 2017 if his Conservatives win a national election next year.

Mr Johnson, often touted as a potential future Conservative leader, is set to outline how Britain should renegotiate its relationship with the EU, it was reported yesterday.

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He is expected to back a report by his chief economic advisor Gerard Lyons into the future of the capital's economy which shows leaving the 28-member EU would not leave Britain significantly worse off.

"Britain can only achieve meaningful EU reform if it's serious about leaving," Mr Lyons was quoted as saying. "Our detailed study shows it's definitely a viable option for the UK to be outside the EU." While British business lobbies have said the EU needs reform, many are worried an EU exit may close British businesses off from the 500 million person single market.