The campaign to remain in Europe is supported by an "extraordinary alliance" of politicians, parties and trade unions, David Cameron has claimed as he took the fight to stay in the EU on the road.

The Prime Minister was joined by former trade union leader Sir Brendan Barber as he unveiled two posters explaining why Britain should remain in the EU.

Mr Cameron conceded the pair disagreed on a lot of issues, but said: "On this issue, this vital issue, we are completely united and say Britain is stronger and better off in a reformed European Union."

The Prime Minister arrived alongside Sir Brendan, the ex-general secretary of the Trades Union Council (TUC), for a tour of the Caterpillar engine factory in Peterborough and took part in a question-and-answer session with hundreds of workers.

Mr Cameron has also penned a letter in the Guardian with Sir Brendan, in which the pair claim that UK workers would be hit by a "triple threat" to jobs, wages and prices if the country votes to leave the EU.

Sir Brendan's successor at the TUC Frances O'Grady has also intervened in the EU debate, warning that "some of the biggest cheerleaders for Brexit see protections for ordinary British workers - like health and safety law - as just red tape to be binned".

An energetic Mr Cameron told workers: "It's not often you find a Conservative prime minister and the leader of a trade union movement standing together, but we both think this issue about Britain and Europe is so important that we put aside our own disagreements, put aside party political arguments to say Britain should stay in a united European Union."

He called the support to remain in EU between the Conservatives, Labour, the Lib Dems and Green Party an "extraordinary alliance and coalition".

The Prime Minister also spoke about the importance of Europe in moving on from the First and Second World Wars.

He said: "I will never forget that 70 years ago the countries in Europe who we sit round the table with were fighting and killing each other for the second time in a century.

"So for all its imperfections we shouldn't lose that idealism that we've found in a way in Europe of settling our differences through discussion and negotiation rather than all the things that have happened in the past."

The intervention by the senior trade unionists came amid claims that the Government had watered down its crackdown on unions in order to secure support in the referendum battle.

Speaking in the House of Commons, leading Tory Eurosceptic Bernard Jenkin said it amounted to "the sale of Government policy for cash and political favours" and put the Government at the "rotten heart" of the European Union.

Downing Street insisted the EU referendum is a "separate issue" to the Trade Union Bill but Labour MPs also suggested Mr Cameron was backing down in his battle with the unions over issues including e-balloting and political funding to ensure their support for remaining in the EU.

Former minister Kevan Jones said: "The climbdown has got nothing to do with the Trade Union Bill, it's to do with the realisation on behalf of the Prime Minister that he wants millions of trade unionists to vote yes in the EU referendum, he's going to have to keep them onside."

Vote Leave condemned the article by Mr Cameron and Sir Brendan. A spokesman said: "Two members of the political establishment doing down the British economy is nothing to be proud of."

And Fire Brigades Union leader Matt Wrack said Sir Brendan's intervention was "appalling", adding: " The former general secretary of the TUC has lined up with a Tory Prime Minister to support Cameron's case for a Remain vote. This is the same Prime Minister who is responsible for unprecedented cuts that have attacked workers' pay, pensions and working conditions."