The UK's workers would be hit by a "triple threat" to jobs, wages and prices in shops if the country votes to leave the European Union, David Cameron and former trade union leader Sir Brendan Barber have warned.
The Prime Minister and the ex-Trades Union Congress (TUC) general secretary acknowledged they were unlikely allies but the "special circumstances" of the EU referendum meant the "rules of conventional politics" could be temporarily set aside.
They warned that Brexit would leave the UK "a poorer country in every sense" but a vote to remain in the EU on June 23 would result in a "bright future" with more jobs, higher pay and lower prices.
In a joint article in The Guardian, the pair highlighted analysis by accountancy giant PwC which suggested unemployment could rise to 8% if there was a Brexit, compared to 5% if the country stays in the EU.
Even if people kept their jobs "their wages will be lower than they would otherwise have been" because of the damage to productivity, while workers' rights would also be jeopardised.
They wrote that "while the two of us may disagree about quite how far this process should go, being in Europe has helped to deliver many of the crucial rights that underpin fairness at work".
Brexit would also weaken the pound meaning "more expensive goods and higher inflation", they said.
"So we are likely to see the cost of living going up, just as wages are being squeezed and jobs lost. That is the threat for families already struggling to make ends meet - and a risk that working people and the poorest in our country simply cannot afford."
The pair took a swipe at Vote Leave figurehead Michael Gove, who has suggested the UK could enjoy a trading relationship with Europe similar to that of Albania, and mocked Brexit campaigners' "conspiracy" theories.
They wrote: "The choice before us is becoming clearer each day. We can risk jobs, hold down wages and pay higher prices. We can be the first major economy in history to deliberately choose a second-rate trading arrangement for our biggest market.
"We can take needless risks with our economy that would unleash a triple threat to working people - and become a poorer country in every sense.
"Or, if we choose to stay in the EU, we can protect working people and the poorest families. We can choose stability and economic security - a bright future with more jobs, higher pay and lower prices."
The Leave campaign had "a real lack of credible experts to support their case" and there was a "troubling dogma" that the risk of Brexit will be worth the pain.
"The leavers now say they want the UK to be like Albania, which even the Albanian prime minister says is "weird", because his country wants to be more like us," they said.
"On our side, however, we have a collection of independent experts, trustworthy organisations and friends of Britain from around the world. Whether it is the Bank of England, our universities, the trade unions, employers large and small in every part of our economy, the IMF, President Obama, our allies in Nato or the Commonwealth, the message is the same: Britain is better off in Europe.
"Of course, the leavers say this must be some sort of conspiracy masterminded by shadowy international elites. All we have to say is: to have been able to bring even the two of us together today, these evil geniuses must be very good."
Sir Brendan's successor at the TUC Frances O'Grady 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".
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.
Downing Street insisted the EU referendum is a "separate issue" to the Trade Union Bill but Labour MPs 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."
Pro-Brexit Employment Minister Priti Patel will use a speech in the City of London to promise cuts to red tape if the country votes to leave.
"We could confine to history the bureaucratic excesses of Brussels," she will say.