A group of disaffected Labour MPs is preparing to quit the party and form a breakaway movement on the political centre ground amid growing discontent with Jeremy Corbyn’s leadership on Brexit and other key issues including immigration, foreign policy and antisemitism.

The Observer has been told by multiple sources that at least six MPs have been drawing up plans to resign the whip and leave the party soon. There have also been discussions involving senior figures about a potentially far larger group splitting off at some point after Brexit, if Corbyn fails to do everything possible to oppose Theresa May’s plans for taking the UK out of the EU.

On Saturday night, three of the MPs widely rumoured to be involved in the plans for an initial breakaway – Angela Smith, Chris Leslie and Luciana Berger – refused to be drawn into talk of a split, and insisted they were focused on opposing Brexit. But they did not deny that moves could be made by the spring or early summer.

Meanwhile, Brexit was being blamed for playing an “inevitable role” in the reported decision by Nissan to abandon plans to build its X-Trail model at its Sunderland plant.

According to Sky News, the company will confirm cancelling plans to build the new version of the SUV on Monday, just 53 days before Britain is scheduled to leave the EU. Sunderland Central Labour MP Julie Elliott said: “The constant uncertainty, the chaotic government. None of it is conducive to encouraging business investment in this country.”

Leslie described rumours of a breakaway as “speculation” but said: “A lot of people’s patience is being tested right now. I think there are some questions we are all going to have to face, especially if Labour enables Brexit.”

Berger said she was focused on Brexit but made clear she, as parliamentary chair of the Jewish Labour Movement, was increasingly angry at the apparent failure of the party to act on complaints of antisemitism inside the party.

While the detailed plans for a breakaway, and the timing of any announcement, are not yet finalised, it is understood that MPs behind the move hope that moderate Tory MPs and Liberal Democrats could join them over time, allowing a new party to be formed that would appeal to voters who no longer feel adequately represented.

Unrest within Labour – and talk of the party fracturing – will be fuelled by the latest Opinium poll for the Observer today, which shows a dramatic fall in its support at a time of chaos and turmoil in the Tory party.

Labour is now seven points behind the Conservatives, with approval ratings for Corbyn over his handling of Brexit at an all-time low and worse than those for May.

The Conservatives are on 41%, up four points compared with the previous Opinium poll on 16 January, while Labour has plummeted by six points to 34% despite the Tories’ travails. The survey suggests Labour is losing its Leave supporters to the Tories and Remain backers to the Liberal Democrats, who are up one point on 8%, as the Brexit crisis deepens.

May’s approval rating on Brexit has edged up slightly to -30% (25% approve against 55% who disapprove) from -33% a fortnight ago. Corbyn’s rating on Brexit is at -44% (16% approve against 6o% who disapprove) down from -40% in the previous poll.

Unrest inside Labour and frustration at its failure to mount stronger opposition to Brexit – and back a second referendum – has grown after a week in which nine shadow ministers were among 25 Labour MPs who escaped any punishment by the leadership after they defied a three-line whip and refused to back an amendment aimed to delay Brexit by up to nine months. The amendment tabled by Labour’s Yvette Cooper was defeated by 321 votes to 298 with the help of the Labour rebels.

Many Labour MPs who back Remain or a soft Brexit said it appeared the leadership had been content for backbenchers to rebel, as Corbyn was not himself truly committed to delaying Brexit.

There is also mounting anger across the party – including within the unions and the grassroots movement Momentum – at the way Len McCluskey, the general secretary of Unite and a key ally of Corbyn, has been holding talks with May and senior Tory ministers in the hope of securing deals on workers’ rights and more cash for industrial areas.

Downing Street hopes a package of concessions aimed at winning over Labour MPs, involving short-term funding for Leave-voting constituencies in the north, will be sufficient to persuade more backbenchers in Leave seats to vote for May’s deal.

Smith insisted her focus was also on Brexit but she tore into the way that McCluskey was apparently being allowed to run a parallel policy on Brexit. “It is outrageous that a trades union leader close to Jeremy Corbyn should be contemplating facilitating Brexit by doing deals with Theresa May in Downing Street which completely undermine the TUC and Labour party position,” she said.

A senior union source said McCluskey seemed to be trying to bypass Labour party and TUC policy, which was to insist on a customs union and close involvement in the single market. “History will damn the treacherous moves to sideline the rest of the trade union movement and frustrate Labour party conference policy,” said the union official.

“He and his allies in the leader’s office haven’t made it a secret that they want Brexit to happen, but to do a secret deal behind the movement’s backs to get it done is shocking even for them.”

A Labour source last night said that a breakaway party would be rejected by voters. “A new party to protect the status quo will rightly be seen as an establishment stitch-up. Any new party set up to promote cuts, corporate control and privatisation will be strongly rejected at the polls.”

Last week there was uproar in the party after it initially suggested it would not oppose Tory plans to end freedom of movement for EU citizens – only to be forced into a U-turn by an outcry among members. There has also been dismay at Corbyn’s refusal to condemn the Venezuelan regime led by socialist president Nicolás Maduro as the country descends into violent chaos.Maduro stands accused of human-rights violations following a crackdown on opposition protesters that intensified after a widely-disputed vote gave his ruling party near unlimited powers at the end of July.

Tomorrow, a fresh row is expected in the party over the handling of antisemitism, with many MPs, including Berger, demanding to know what progress is being made into complaints the party is investigating. Berger is insisting on the party revealing the full details within a week.

The Observer has also learned of mounting anger on the former solidly pro-Corbyn left of Labour. Ana Oppenheim, an organiser for the leftwing anti-Brexit movement Another Europe is Possible, who is also active inside the pro-Corbyn grassroots movement Momentum, suggested any Labour MPs who refused to do all in their power to block May’s Brexit should be deselected.

“This is an existential moment for the left,” she said. “The Tory Brexit deal will mean a huge expansion of border controls and will open us up to unprecedented economic deregulation. If Labour MPs cannot unite to block it, they shouldn’t be Labour MPs. Members have mechanisms to choose MPs who actually represent the Labour movement, and we would encourage them to use these.”