Jeremy Corbyn is under huge pressure to shift party policy on Brexit as an exclusive poll for the Observer reveals a substantial majority of existing and potential Labour voters want him to back permanent membership of the EU’s single market and customs union.

Four times as many Labour supporters favour that option as oppose it. The survey by Opinium also finds that more than twice as many in this group want Corbyn to support a second referendum on the eventual Brexit deal as reject it.

The poll comes at a time when senior European politicians are signalling that Britain could reverse its decision to leave the EU and warning that there will be no prospect of reaping the benefits of membership from outside the bloc.

Emmanuel Macron will say on Sunday on The Andrew Marr Show that he would love to see Britain return, adding: “It depends on you. I do respect this vote, I do regret this vote, and I would love to welcome you again.” However, he warns that full access to the single market for the UK’s huge financial services industry will be “not feasible”.

Play Video 1:06 Emmanuel Macron: special deal is possible for UK – video

Meanwhile, Italy’s minister for European affairs, Sandro Gozi, said that the Brexit decision could be reversed. He told LBC radio that “by definition, Britain cannot have the benefits of remaining in the union” while being outside it.

The CBI is preparing this week to call on the government to abandon the pursuit of global trade deals in favour of long-term EU customs union membership. In the wake of the Observer poll, senior Labour MPs are saying it is no longer credible for a party that is serious about power to overlook the demands of so many trade unions, supporters and business leaders.

The former shadow business secretary, Labour MP Chuka Umunna, said: “It is absolutely vital, as the party of work and social justice, that we have a clear and unequivocal policy to keep the UK in the single market and the customs union.

“When our members, our trade unions, our businesses, Labour voters and potential voters are all giving the Labour party the same clear message that this is what the country needs, it does not make any sense at all to ignore it.”

Opinium found that among all potential Labour voters at the next election, 56% want the party to back permanent membership of the single market and customs union, while just 13% think it should remain opposed; 30% had no firm view either way.

Among younger voters, the balance in favour was even clearer: 63% of all “young professionals” who are potential Labour backers were in favour, against 10% who were opposed. Of the “young blue collar workers” who might back Corbyn’s party, 61% were in favour of the change against just 4% who were against.

Significantly, even among Labour voters who backed Leave, 37% said they want Corbyn to support permanent single market and customs union membership compared with just 26% who did not.

Some 51% of Labour’s potential pool of support backs a second referendum on the final Brexit deal, against 23% who oppose the idea.

Despite mounting disquiet among pro-Remain Labour MPs, Corbyn has so far rejected calls to back staying in the single market and customs union. Remaining as full participants in both would require the UK to abide by EU rules on free movement of people and workers, and to submit to the jurisdiction of the European Court of Justice on trade issues.

Labour’s current policy is to keep the UK in both only for the duration of a post-Brexit transition of up to four years. After that it would try to negotiate a relationship offering the “exact same benefits” for the UK post-Brexit. Corbyn has also refused to endorse a second referendum.

Overall, most voters think that both the Labour and Tory positions on Brexit are unclear. Only a quarter (26%) think Labour’s position is clear, while 60% think its position is unclear. The Conservatives – who are committed to taking the UK out of the single market and customs union after a transition period – fare only slightly better. Just over a third (36%) think the Tories are clear on Brexit while just over half (53%) think their position is unclear.

Another strongly Remain Labour MP, Chris Leslie, said there had been a marked shift in opinion among grassroots Labour supporters. “MPs are increasingly responding to views of Labour supporters,” Leslie said. “I am increasingly convinced that Labour will soon back staying in the customs unions and the single market.”

In a speech on Monday, Carolyn Fairbairn, director general of the CBI, will say that the benefits of signing new trade deals with the United States and others are far outweighed by maintaining good links to the EU.

She will propose that a comprehensive customs union between the UK and the EU would be a “practical, real-world answer” that would help solve many of the trading and Irish border issues created by Brexit.

“The idea behind a customs union is simple: a single set of tariffs for goods imported from outside the EU, enabling tariff-free trade within it,” she will say. “It brings no obligations over freedom of movement, or payment, and removes some of the heaviest trade barriers. Importantly, it would go a long way towards solving the border problem in Ireland.

“We recognise this isn’t easy, and of course it’s worth looking at the alternatives. Yet unless and until we find one that works, a customs union is part of a practical, real-world answer.

“It needn’t stop us keeping an eye on our global trade interests. There may come a day when the opportunity to fully set independent trade policies outweighs the value of a customs union with the EU. A day when investing time in fast-growing economies elsewhere eclipses the value of frictionless trade in Europe. But that day hasn’t yet arrived.”

The poll results