JEREMY Corbyn will today insist there is a "strong socialist case for staying in" the European Union as he makes his first significant speech of the referendum campaign.

The Labour leader, who will insist Labour is "overwhelmingly" in favour of Britain remaining in the EU, has come in for criticism that his stance on the EU debate has been, thus far, lukewarm; during his party’s leadership contest last year he did not rule out backing Brexit.

Pointing to serious "shortcomings" in Brussels, he will claim the Conservatives, not the EU, were responsible for the crisis in the British steel industry and a failure to tackle tax avoidance and that Labour would work with continental allies on such issues.

Mr Corbyn will base his argument for staying in on the protection of workers' rights, strict environmental standards and the protection of consumers from rip-off charges.

"The Labour party is overwhelmingly for staying in because we believe the European Union has brought investment, jobs and protection for workers, consumers and the environment, and offers the best chance of meeting the challenges we face in the 21st century," the Labour leader will say in a speech in central London.

Issues such as climate change, corporate tax dodging, cyber-crime, terrorism, protecting jobs and pay from the pressure of globalisation, the refugee crisis and ageing populations "self-evidently require international co-operation", he will say.

"Collective international action through the European Union is clearly going to be vital to meeting these challenges. Britain will be stronger if we co-operate with our neighbours in facing them together."

Mr Corbyn will acknowledge that he remains "critical of its shortcomings, from its lack of democratic accountability to the institutional pressure to deregulate or privatise public services".

"So Europe needs to change. But that change can only come from working with our allies in the EU. It's perfectly possible to be critical and still be convinced we need to remain a member."

The party leader will warn against the temptation "to blame the EU, or worse to blame foreigners" for the country's problems when the issue was "a Conservative government that is failing the people of Britain".

He will argue: "There is a strong socialist case for staying in the European Union, just as there is also a powerful socialist case for reform and progressive change in Europe.

"But we also need to make the case for reform in Europe; the reform David Cameron's government has no interest in but plenty of others across Europe do.”

Mr Corbyn’s speech comes after the Electoral Commission, the official elections watchdog, said it had selected Vote Leave and Britain Stronger in Europe as the designated official Leave and Remain campaigns in June's EU referendum.

Vote Leave - backed by London mayor Boris Johnson and Justice Secretary Michael Gove - saw off a challenge from a rival campaign Grassroots Out, backed by UKIP leader Nigel Farage.

The campaigns will be allowed to spend up to £7 million, get a free mailshot, TV broadcasts and £600,000 public funds.

The commission said the Vote Leave campaign had been chosen as it had "well-developed plans and structures" to work with other pro-Brexit groups in the run up to the vote on June 23.

However, rival Leave.EU - the group founded by wealthy Ukip backer Arron Banks, which was supporting the application by the rival Grassroots Out campaign - swiftly announced it would seek a judicial review.

After consulting lawyers, Mr Banks warned the move could force the UK Government to put back the date of the referendum to October

"I am thoroughly unsatisfied with the Electoral Commission's decision for a variety of reasons that I will be making clear in my application for judicial review," he said.

However, in a terse one-sentence statement issued just 40 minutes later, Leave.EU said it would make a decision on whether to proceed with a judicial review by midday today. There was no explanation for the apparent change in position.