Opposition MPs say they have agreed to try to block a no-deal Brexit by passing legislation in Parliament.
A meeting between those opposed to no deal was called by Jeremy Corbyn.
A joint statement afterwards did not detail how or when legislative attempts would be made, but Green MP Caroline Lucas said it was the best way to stop a PM "careering towards" no deal.
The statement also made clear using a vote of no confidence to bring down the government remained an option.
The UK is set to leave the EU by 31 October and Boris Johnson has promised to stick to that date even if he cannot agree a deal with Brussels.
Ms Lucas said "the legislative way forward" was "the most secure way to try to extend Article 50, to get rid of that 31 October deadline".