Jeremy Corbyn is facing renewed calls to commit Labour 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.
Shadow Brexit secretary Sir 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.
However the signatories to the 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 Lord Hain and Liverpool mayor Joe Anderson, said Labour needed to present an alternative to the Tories' "destructive Brexit".
"The supposed benefits of a clean break with the EU are a fantasy. The economic impact of leaving the single market would hit the most vulnerable in our society hardest," the letter said.
"So 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.
"This would send a powerful message of solidarity to the rest of Europe, and to the millions of EU and UK nationals living in limbo here and across the continent."
The Labour leadership has so far resisted calls to remain in the single market permanently which would entail accepting free movement of labour - a move it fears would be deeply unpopular in many of its traditional heartlands where immigration is a key concern among voters.
Meanwhile shadow chancellor John McDonnell has sought to put pressure on the Conservatives over university tuition fees, offering to back any move by Chancellor Philip Hammond to raise the £21,000 salary threshold at which graduates have to start repaying their loans.
"What I am saying to him is if he comes forward with effective proposals on thresholds and interest rates then we will support him. It has got to be a significant step," he told The Sunday Times.
"Because the Tories increased fees from £3,000 to £9,000, it has pushed the system into implosion. Seven out of 10 students aren't going to pay it back so someone else has got to sort the system out because it's imploding around our ears."
In an article for The Observer, Mr Corbyn underlined the mood of confidence in the Labour leadership after the party's unexpected gains in the June general election when it wiped out Theresa May's Commons majority, saying his party is now a "government in waiting".
"We have changed the political centre of gravity. We are now the political mainstream and have the chance to transform our country," he wrote.
"To do that we must use our new strength inside and outside parliament to challenge the Conservatives at every step - and prepare to form a government to change Britain when the next election is called."