BIRMINGHAM, England (Reuters) – Britain’s opposition Labour Party on Monday demanded that parliament have the final say on the government’s Brexit deal, including an option to send ministers back to the negotiating table rather than leave without an exit agreement.

After reaching a deal last week on a transitional period lasting until the end of 2020, Prime Minister Theresa May’s team must now negotiate Britain’s long-term trading arrangements with the European Union – perhaps the hardest stage yet in the complicated divorce talks.

The government has promised to put that final deal to parliament for approval, but has made clear the choice is either to accept the exit agreement, or leave without a deal.

On Monday, Labour’s Brexit policy chief Keir Starmer called for a different approach.

“If Parliament rejects the terms of the Prime Minister’s deal that would not give her licence to crash out without an agreement. Far from it: that would be the worst of all possible worlds,” he said in a speech in Birmingham.

In a separate speech, former Labour prime minister Tony Blair, whose centrist views are at odds with the current left-wing leadership of Jeremy Corbyn, will urge parliament to go further and insist on a referendum on accepting the deal.

A so-called ‘No Deal’ exit would likely cause upheaval on financial markets, throw cross-border trade into confusion, and spark a political crisis in the world’s sixth largest economy.

Labour will seek to give parliament more options — including a return to Brussels for fresh talks — by amending the legislation that will end Britain’s EU membership on March 29, 2019.

“The amendment will make clear that – should the prime minister’s proposed Article 50 deal be defeated – it would then be for parliament to say what happens next, not the executive,” Starmer said.

The government argues that leaving open the possibility of more talks means that EU negotiators won’t give Britain the best deal possible during the current negotiations.

The European Union (Withdrawal) Bill has tested May’s ability to rule, as her Conservatives remain divided on the best approach to Brexit and she is dependent on support from a small party in the province of Northern Ireland for a majority.

In December she was defeated in parliament when 11 members of her party said her promise of a vote on the final deal had to be guaranteed in law.

Labour’s own internal divisions on Brexit were exposed on Friday when Corbyn sacked his shadow Northern Ireland minister after he called for a second referendum on Brexit.

Later on Monday, Blair will argue that the government will fudge the terms of its final deal to ensure Britain actually leaves the bloc.

“Basically, we will have to take what we are given. By the end of 2020, the transition will end. The cliff edge will beckon. We can navigate a harder or easier descent; but retreat will be impossible,” he will say.

Blair has made several interventions seeking to build support for reversing Brexit, saying it will be regretted for generations.

“Only parliament can change the direction of this process. Only parliament can give back to the people the final say on the terms the government negotiate,” he will say.