Jeremy Corbyn has warned that a no-deal Brexit will put the UK “at the mercy of Donald Trump”.
The Labour leader said leaving the European Union without an agreement is “really a Trump-deal Brexit”.
He accused prime minister Boris Johnson of “cosying up to Trump” at the recent G7 summit in France.
Mr Corbyn held talks with cross-party figures today in an effort to unite around a plan to block a no-deal Brexit.
Anna Soubry, leader of the Independent Group for Change, tweeted afterwards: "Excellent meeting between all the opposition party leaders this morning. We agree we will work together to stop a no deal #Brexit by legislation."
SNP Westminster leader Ian Blackford said the meeting was "positive and productive".
He said: "With no mandate or majority, the numbers are stacking up against Boris Johnson and his extreme Brexit. The possibility of stopping Brexit is real and must be realised.
Excellent meeting between all the opposition party leaders this morning. We agree we will work together to stop a no deal #Brexit by legislation
— Anna Soubry MP (@Anna_Soubry) August 27, 2019
"Parliament must grasp this opportunity, unite to stop Boris Johnson shutting down democracy - and be ready to use all mechanisms to block a no-deal disaster, including deploying legislation as a priority.”
Meanwhile, shadow Brexit secretary Sir Keir Starmer said Labour would campaign for Remain in the event of a second EU referendum.
On Tuesday, he told BBC Radio 4’s Today programme: “The only way out now is to have a referendum, to ask the people whether they're prepared to continue down this path.”
Mr Johnson said he was "marginally" more optimistic about securing a deal, but insisted other EU states needed to shift ground.
Writing in the Independent on Tuesday, Mr Corbyn said: "The Tories are going out with their begging bowl to billionaire hedge funders to raise cash for an autumn general election.
"The chaos and uncertainty caused by a no-deal Brexit is a potential goldmine for speculators betting against the pound.”
He warned a no-deal Brexit “won't return sovereignty, it will put us at the mercy of Trump and the big US corporations dying to get their teeth into our NHS, sound the death knell for our steel industry and strip back our food standards and animal welfare protections”.
Mr Corbyn insisted his party will do “everything necessary” to block a no-deal Brexit as he meets other senior MPs to try to find a way to challenge Mr Johnson over EU withdrawal.
Before discussions on Tuesday with leading Commons figures opposed to breaking EU ties without an agreement in place, Mr Corbyn insisted the country was heading for a crisis under Mr Johnson.
A no-deal exit from the EU would amount to a "bankers' Brexit" benefiting the rich, Mr Corbyn said.
"The battle to stop no-deal Brexit isn't a struggle between those who want to leave the EU and those who want continued membership.
"It's a battle of the many against the few who are hijacking the referendum result to shift even more power and wealth towards those at the top.
"That's why the Labour Party will do everything necessary to stop a no-deal bankers' Brexit."
Mr Corbyn said an "injection of democracy" could come through a new referendum, or a general election.
He added: "In that election, Labour will offer a referendum, with a credible leave option as well as the option to remain.
"But we won't rule out other options that could stop this no-deal disaster in its tracks. I'll discuss all these options with the leaders of other opposition parties.
"I hope we can come to a good working arrangement and bring on board others across Parliament who see the danger of a no-deal crash-out."
The comments came as the Guardian reported that legal advice prepared for the Labour leader by shadow attorney general Shami Chakrabarti claims that Mr Johnson would be committing the "gravest abuse of power and attack on UK constitutional principle in living memory" if he shut down parliament to force through a no-deal Brexit.
Among those attending the cross-party talks on Tuesday is Liberal Democrat leader Jo Swinson.
Ms Swinson, and other prominent opponents of a no-deal Brexit, have said Mr Corbyn would not be the right candidate to lead a short-term national government if the Commons forced Mr Johnson out of Number 10.
Opposition leaders attending the discussion with Mr Corbyn on parliamentary tactics include SNP Westminster leader Ian Blackford, the Independent Group for Change's Anna Soubry, and Plaid Cymru Westminster head Liz Saville-Roberts.
James Cleverly, chairman of the Conservative Party, said: "Jeremy Corbyn's Labour Party will do anything to try and block delivering the change that British people voted for in the referendum.
"The alternative to delivering Brexit is Jeremy Corbyn in Downing Street; a man who would wreck the economy, break up our Union, is soft on crime and won't stand up for Britain. All Corbyn offers is chaos, delay and uncertainty."
Mr Johnson insisted the UK will leave the EU on October 31 with or without a deal, but faces Commons clashes over the issue in the coming weeks.
It is understood that Mr Johnson is seeking to appoint Brexit-supporting peers to the House of Lords in a bid to get more support for the government's stance on Europe in the upper chamber.