Labour has been forced to take a significant step towards keeping Britain in the EU single market because of a line snuck into a motion agreed by the party’s conference last week, trade unions have claimed.

Union leaders and pro-EU Labour activists claimed a significant victory as they insisted the wording of the Brexit policy approved by the conference would force Jeremy Corbyn and his top team to adopt a position they had previously ruled out.

The motion passed by delegates at the gathering in Liverpool said Labour should support “full participation in the single market”.

That is a significant shift from the party’s current policy, which says only that Labour would seek “full access” to the single market, according to trade union leaders and anti-Brexit campaigners who drafted the line.

They suggested it would force the Labour leadership to commit, as a minimum, to keeping the UK in the European Economic Area (EEA) – in effect meaning membership of the single market.

The Labour leadership has previously dismissed this option, saying it is not right for the UK and would leave Britain as “rule-takers not rule-makers”. It is also opposed by many of the party’s MPs.

“Full participation” in the single market would also likely mean a continuation of free movement after Brexit, they said – another policy the Labour leadership has previously ruled out.

A Labour spokesperson denied the policy had shifted, saying the wording of the motion was “in line” with the party’s plan.

That raises the prospect of a major row between the Labour leadership and some of its key trade union allies, with the two having already offered very different interpretations of its meaning.

Manuel Cortes, a key supporter of Mr Corbyn and leader of the TSSA transport workers union that proposed the change, told The Independent: “It is now Labour policy that we must have full participation in the single market, not just access.

“We must now fight for a deal which a minimum delivers this, a customs union and Labour’s six tests as well as keeping ‘Remain’ on the table.”

And a spokesperson for another union, Community, said: “With over half of our manufacturing exports going into the single market, we were pleased to see Labour shift its policy in favour of ‘full participation’, which would surely mean EEA membership.

“This is vital to protecting the jobs of Community members working in manufacturing. Labour took a step forward at their conference by calling for the people to have a final say on Brexit, and they must continue to press the government on the importance of both the customs union and the single market.”

The crucial part of the motion reads: “Conference believes we need a relationship with the EU that guarantees full participation in the single market. The Brexit deal being pursued by Theresa May is a threat to jobs, freedom of movement, peace in Northern Ireland and the NHS.”

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The wording was hammered out during a tense six-hour meeting of delegates that ran well into the evening of the first day of the conference.

While the discussions were dominated by prolonged debate about Labour’s policy on a fresh Brexit referendum, campaigners believe the reference to the single market that was agreed without major discussion is more significant than many in the room realised.

A spokesperson for the Another Europe is Possible group, which wants Labour to soften its Brexit stance and played a pivotal role in driving through the wording of the motion, said: “Staying in the single market ought to mean that an EEA option is Labour’s baseline.

“This should preserve free movement and many of the rights and protections currently enshrined in EU law.

“We just wouldn’t have any say in making the laws we’d have to live by – which is why staying in the EU is a straightforwardly better option.”

Labour votes to keep open the option of second Brexit referendum

Participation in the single market would almost certainly mean a continuation of freedom of movement, which Labour has promised to end.

The motion also criticises Theresa May’s planned deal for being “a threat to jobs, freedom of movement, peace in Northern Ireland and the NHS” – a significant phrase given Labour is also committed to ending free movement.

Activists claimed this marked a second significant shift in the party’s position.

The Labour Campaign for Free Movement said the conference had “overwhelmingly voted through a policy committing the party to defending the free movement of people with Europe”.

It added: “The proposal also seeks ‘full participation’ in the single market, which is not possible without free movement of people.”

That is likely to infuriate some Labour MPs who have argued strongly that the party must not support anything that would keep Britain in the single market or allow freedom of movement to continue.

Gareth Snell, MP for Stoke-on-Trent Central, told The Independent: “Any shift in Labour policy that doesn’t honour our manifesto commitment to both leave the EU and end freedom of movement would cause real electoral problems for Midlands and heartlands constituencies and would allow the opposition to paint the Labour Party as not honouring the result of the referendum.

“The wording of the motion is trying to be all things to all people. It is dangerous because unless there is one clear Labour narrative, it’s difficult to defend what we are doing. It neither helps me in Stoke-on-Trent nor Ben Bradshaw in Exeter [a heavily Remain area] to have a muddied policy on this. We need clarity on freedom of movement and to show that we accept the result of the referendum.”

Labour denied that the wording marked a shift from its current position.

A spokesperson said: “The motion passed at conference is in line with Labour’s plan for a strong single market relationship, with shared institutions and common standards, rights and protections.