Labour leader Jeremy Corbyn is being pressed to commit to remaining in the European single market and customs union after the UK leaves the EU.
More than 40 senior Labour figures, including 30 MPs, have signed an open letter published in the Observer.
They say the party must offer a clear alternative to the Conservatives’ “destructive Brexit”.
The first full day of the Labour party conference will begin in Brighton later.
Although Labour did not win the general election, Mr Corbyn will tell delegates to the party’s biggest-ever conference that they have set the political agenda after making gains in it.
He will unveil, or reiterate, policies that will have wide approval across his party, from recruiting more police officers to lifting the public sector pay cap and alleviating student debt.
“We have changed the political centre of gravity. We are now the political mainstream and have the chance to transform our country,” he wrote in the Observer.
“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.”
On Brexit, he said Labour was making the case for a “jobs-first Brexit that prioritises access to European markets”.
The conference will debate Brexit policy on Monday.
The letter urging commitment to the single market was signed by both Blairite and left-wing MPs, along with some prominent trade union leaders.
“The supposed benefits of a clean break with the EU are a fantasy,” it said.
“The economic impact of leaving the single market would hit the most vulnerable in our society hardest.
“Labour should commit to staying in the single market and customs union – ruling out no options for how to achieve this.”
Mr Corbyn has so far resisted committing to the single market and the customs union after Brexit.
But Shadow Brexit secretary Sir Keir Starmer has previously said that under a Labour government Britain would remain in the single market and a customs union for a transitional period after Brexit.
The customs union is the EU’s tariff-free trading area, which imposes the same taxes on imports from certain countries outside the union.
The single market also includes the free movement of goods, services, capital and people.