EU referendum: Boris Johnson set to back 'In' campaign, reports claim

If confirmed, the Mayor of London’s decision would come as a further blow for the embattled Brexit campaign, which has struggled to find a leader amid squabbling between rival groups

Many had expected the London Mayor to join the Brexit campaign (EPA)

Boris Johnson – seen by Brexit campaigners as a political big hitter who could persuade voters to support leaving the European Union – is set to back remaining within the 28-nation bloc instead, according to reports.

The Guardian and Financial Times newspapers both said that the charismatic Mayor of London and Conservative MP for Uxbridge and South Ruislip was expected to support the Prime Minister.

If confirmed, Mr Johnson’s decision would come as a further blow for the embattled Brexit campaign, which has struggled to find a leader amid squabbling between rival groups.

A source in the pro-EU campaign told the Financial Times: “The one person who could galvanise the Out campaign would be Boris: he’s the one we really fear.

“Without Boris, the Leave campaign does not have a leader.”

The paper added that David Cameron had promised Mr Johnson a “major cabinet job” after he stands down as London mayor in May.

Mr Cameron has sought to address concerns raised by Mr Johnson about the balance of power between London and Brussels, The Guardian reported.

It said that the Prime Minister was expected to declare that the UK’s Supreme Court, or another official body, had the power to assess whether acts by EU institutions were outwith its powers – similar to Germany’s constitutional court.

Mr Cameron may also propose a new act of parliament, which would assert that the UK could withdraw its agreement to the primacy of EU law.

This move to “put beyond doubt” the sovereignty of the British parliament was privately welcomed by Mr Johnson, the Guardian reported.

However a senior Eurosceptic Tory MP dismissed the measures, telling the paper that they were “like constitutional window dressing”.

Following Home Secretary Theresa May’s warm words for the reforms Mr Cameron negotiated with EU President Donald Tusk, it would appear that former Conservative leader Iain Duncan Smith may be the highest profile politician to back leaving the EU.