Theresa May will be feeling the heat this afternoon as Jeremy Corbyn’s vision for Britain remaining in the EU customs union – set out this morning – draws support from groups across the political spectrum.

In a speech about Brexit his morning, Corbyn put pressure on the prime minister, calling for her Conservative MPs to cross lines and vote with his party, saying:

“I appeal to MPs of all parties, prepared to put the people’s interests before ideological fantasies, to join us in supporting the option of a new UK customs union with the EU”.

10 Downing street re-affirmed this afternoon that the government will not be joining a customs union, saying in a statement: “we want to have the freedom to sign our own trade deals and to reach out into the world.”

With Labour’s position now clearer, Theresa May’s hardline stance risks alienating further both her party’s pro-EU MPs but also the party’s natural allies in the business world.

Carolyn Fairbairn, director general of the CBI – in the past highly critical of Labour’s re-nationalisation policy – for example, welcomed Corbyn’s words saying:

“The Labour leader’s commitment to a customs union will put jobs and living standards first by remaining in a close economic relationship with the EU. It will help grow trade without accepting freedom of movement or payments to the EU.

“Many thousands of ambitious UK firms are looking to break into new markets. These companies need government to focus on making access to markets simpler, not putting up barriers to our most important trading partner.

“Importantly, a customs union will go part of the way to providing a real-world solution to the Irish border question that is of such urgent concern to the people and firms of Northern Ireland.”

Less surprisingly, support from Labour’s allies in the trade unions was also forthcoming. TUC general secretary Frances O’Grady said:

“The Labour leader’s speech was a welcome step forwards. It provided some of the answers working people need on how their jobs, rights and livelihoods will be protected. And it exposed the threat to manufacturing jobs from the government’s red line on a customs union.

“The ball is now in Theresa May’s court. She must explain how her government will safeguard the Good Friday Agreement and keep Northern Ireland free from a hard border. And she must explain how people’s jobs will be kept safe if we no longer have the benefits of a customs union.”