British Labour leader Jeremy Corbyn is resisting pressure from Labour MPs to commit the party to keeping the UK in the EU single market after Brexit.
As activists gathered in Brighton for the start of Labour's annual conference, 30 senior figures have written an open letter calling for the party to do whatever it takes to keep Britain in the single market and the customs union.
But in a high-profile TV interview with BBC1's Andrew Marr Show kicking off the week's events, Mr Corbyn said Labour needed to be careful not to give up powers which it will need in government to deploy state support for industry.
Shadow Brexit secretary Keir Starmer has previously said that under a Labour government Britain would remain in the single market and the customs union for a transitional period of two to four years after Brexit.
But signatories to a letter published in the Observer, including former shadow cabinet members Chuka Umunna and Heidi Alexander, as well as one of Mr Corbyn's closest allies in his early days as leader, Clive Lewis, said the party should go further to protect jobs and workers' rights.
The letter, which was also signed by the TSSA union's general secretary, Manuel Cortes, former Northern Ireland secretary Peter Hain and Liverpool mayor Joe Anderson, warned that leaving the single market would "hit the most vulnerable in our society hardest".
"At our conference this week, Labour should commit to staying in the single market and customs union - ruling out no options for how to achieve this - and to working with sister parties and others across Europe to improve workers' rights, boost trade union membership and put an end to the exploitation of workers, not freedom of movement," they said.
Speaking to Mr Marr, Mr Corbyn said he wanted to ensure "tariff-free access to the European market".
But he added: "I would also say that we need to look very carefully at the terms of our trade relationship, because at the moment we are a part of the single market and that has within it restrictions on state aid and state spending and pressures on it, through the European Union, to privatise rail and other services.
"I think we need to be careful about the powers we need as a national government."
He suggested that EU rules could have prevented him as prime minister from intervening to prop up Britain's steel industry during its recent crisis, and would block a future Labour government from investing in industries.
Meanwhile, the leader of the Irish Labour Party Brendan Howlin is attending the conference in Brighton, and will focus his efforts on seeking to further develop the Labour Party position on Brexit.
"As members of the Party of European Socialists, I have been working alongside Jeremy Corbyn and Colum Eastwood over recent months to try to influence the debate on Brexit in the United Kingdom, and to clearly spell out the dangers of Brexit to the island of Ireland.
"I will be reiterating my belief that Brexit will be bad for Britain, and bad for Ireland too.
"And I will be welcoming the nuanced position that Labour has adopted over recent weeks, while calling upon the Labour leadership to go further, and to take whatever actions they can to stop the relentless drive towards a hard Brexit," said Mr Howlin.