GORDON Brown has warned that Britain faces a "dog-eat-dog" race to the bottom in workers' rights if it left the European Union.

The former Prime Minister, in his first major intervention in the In-Out campaign to shore up the Labour working class vote, targeted much of a keynote speech at how the EU benefited and would continue to benefit ordinary workers from creating more jobs and limiting the maximum working week to employment protection such as over holiday pay.

“My appeal to the Labour voters is a positive on; that there are benefits for their families on jobs, on action on tax unfairness, on social standards and on tackling breaches of security that have led to terrorism, where we need European co-operation.

“These are the positive arguments that will be more persuasive for Labour voters. So part of the campaign for people like me and Jeremy[Corbyn] and others will be going round the country talking about these positive reasons; how people can see there is a benefit in their lives from being part of the EU and the EU can work for them and Britain can lead in the next stage of the development of the EU.”

Ahead of Mr Brown’s keynote speech at the London School of Economics, a senior Labour figure and a former Cabinet minister stressed how the Scot would play a key role in persuading ordinary Labour voters, under threat of switching to the likes of the anti-EU Ukip, to vote to stay in the EU. He stressed how the Labour working class vote would be “crucial” in the outcome and could be the difference between Britain staying in the EU or leaving.

In a wide-ranging speech – sprinkled with humorous anecdotes – the former premier warned that Brexit would trigger a backlash against employment rights.

"Think of the maximum working week. Think of holiday pay. Think of the transfer of undertakings when companies go bust and employees are protected. Think of the social chapter in Europe preventing a race to the bottom, preventing a dog-eat-dog competition between European nations vying with each other for the inward investment that's available by social dumping and by the lowering of standards.”

Mr Brown insisted the bloc of 28 nations had created something better, meaning that the original customs union, that became a common market, which turned into a single market, had now become a “social market”.

"Underpinning the European Union now is the belief that markets need morals…that markets may be free but they can never be values-free.

"Underpinning the European Union, and the social chapter, and the social policy, is this view that we must prevent a race to the bottom where the good-paying country, or employer, or company, is not undercut by the bad, and the bad is not undercut by the worst.

"This is a vision of Europe that no other continent, neither America, nor Asia, nor the Middle East, nor Africa has managed to achieve; that the market is not simply about trade, it is about rights, economic and social, as well as civil and political rights," declared the former MP.

Declaring that Britain should be part of a “united Europe of states, not a united states of Europe,” he insisted: “Britain is best served by being a leader in Europe, not just be being a member.”

On the issue of migration, Mr Brown argued that the biggest problem was illegal immigration.

“We need the co-operation of security services and domestic agencies right across Europe to deal with this problem.” He said the proposal by the Brexit camp to have ad hoc arrangements with our European counterparts would not be enough and what was needed was a pan-European structure.

In a post-speech Q&A, Mr Brown said he would be happy to debate the pros and cons of EU membership with leading Vote Leave figure Boris Johnson and suggested that the former London mayor was not a believer in the United Kingdom.

He said: “I was part of the Scottish referendum campaign; I’m not so sure Boris is on our side on that either.”

Mr Brown’s intervention - which will be followed by a speech in Birmingham tomorrow and one in the coming days in Scotland - came as Mr Johnson denied that he expected a Leave vote in the June 23 referendum to clear the way for him to succeed David Cameron as Prime Minister.

Asked whether he thought his Tory colleague should remain in No 10 to oversee negotiations for Britain's withdrawal from the EU, the former London mayor told ITV1's Good Morning Britain: "Yes, absolutely. Of course he can, and he must."

Later today in a speech, Liberal Democrat leader Tim Farron will echo Mr Cameron's warning of the threat posed by Brexit to peace and security, arguing that a Leave vote would risk a return to the "mutual hostility" of a century ago, when Europe was convulsed by the First World War.

Kicking off a Vote Leave battlebus campaign in Truro, Cornwall, Mr Johnson said the referendum offered the UK a "once-in-a-lifetime chance for us to take back control of our country and our democracy".

The Uxbridge and South Ruislip MP repeated the Vote Leave claim that the UK sent £350 million a week to Brussels, despite a second warning from the head of the UK Statistics Authority, Sir Andrew Dilnot, who on Tuesday wrote to tell the campaign it was "disappointing" that it continued to use a figure which he regarded as "potentially misleading" and lacking in clarity.

Mr Johnson - widely seen as a likely contender for a Tory leadership battle when the PM stands down - insisted that the Brexit camp could "guarantee" that Britain's EU contributions would be spent on UK priorities like Cornwall's fishing and farming industries if the UK voted to leave.

He told BBC1's Breakfast: "Of the £20 billion we send to Brussels a year, £10bn we never see again. It goes on all sorts of things; Greek tobacco farming, Spanish bull-fighting.

"With that net money back in our country we could fund things like the NHS, our science base, our academic health science centres even more generously than we currently do. That argument just doesn't stack up."

Mr Johnson was not chastened by US President Barack Obama's warning that Britain would be "at the back of the queue" for a trade deal if it left the EU.

He said: "Obviously, when the US wants us to be at the front of the queue for various things - the Iraq War - then that's a different matter.

"Most sensible people will recognise that we will do a free trade deal not just with the EU but we will have the opportunity for the first time in 43 years to do free trade deals not just with America but with India, China, Australia and New Zealand, which we currently cannot do because we are a member of the European Union."

He rejected the claim of Labour's Alan Johnson that Brexit campaigners were "extremists" who could see nothing good in Europe, telling the BBC: "I do think it's very odd that we are being called extremists and irrational when only the other day we were told World War Three was going to break out if we voted to Leave. That cannot be sensible.

"Everybody knows that peace in this continent is really guaranteed by Nato. If it really is true that World War Three and bubonic plague are about to break out, why on earth are we having this referendum?

"I love Europe. I have many happy memories of living, working, going on holiday to Europe. Most of my family come from one European country or another.

"But there's a difference between Europe and the European institutions, and they are now evolving in a way which is not compatible with the long-term health of our democracy."