LONDON — The leader of Britain’s biggest opposition party on Wednesday urged other opposition forces to unite, topple Prime Minister Boris Johnson’s Conservative government, and prevent Britain from leaving the European Union in October without a divorce agreement.

The move came after Johnson accused anti-Brexit UK politicians of collaborating with the EU to stymie Britain’s exit from the bloc.

Jeremy Corbyn, who heads the main opposition Labour Party, said he planned to call a no-confidence vote in Johnson’s government ‘‘at the earliest opportunity when we can be confident of success’’ once Parliament returns from its summer break in September.

In a letter to other opposition leaders and pro-EU Conservative lawmakers, the Labour chief said Parliament should then unite behind a Corbyn-led ‘‘temporary government’’ that would seek a delay to Brexit day — currently scheduled for Oct. 31 — and call a national election.


The plan is feasible under Parliament’s rules, but is likely to face resistance. The smaller opposition parties agree on the need to avoid a no-deal Brexit, but don’t want to put Corbyn — a veteran left-winger whom many distrust — in power. Labour, meanwhile, is likely to oppose a politician from any other party heading a national unity government.

Meanwhile, the man who was Britain’s top finance official until three weeks ago accused Johnson’s Conservative government on Wednesday of steering the country toward a damaging no-deal Brexit that isn’t backed by Parliament or the voters.

Philip Hammond, a Conservative legislator who stepped down as Treasury chief just before Johnson became prime minister last month, said ‘‘leaving the EU without a deal would be just as much a betrayal of the referendum result as not leaving at all.’’

Hammond told the BBC that Johnson had moved from a tough negotiating stance to a ‘‘wrecking’’ one by insisting on changes to the withdrawal agreement between Britain and the EU that the bloc would not accept.


He said that while he believed Johnson wanted a deal, ‘‘there are other people around him whose agenda is different’’ — an apparent reference to advisers such as Dominic Cummings, one of the architects of the country’s 2016 decision to leave the EU.

Johnson has vowed that Britain will leave the EU on the scheduled date of Oct. 31, with or without a divorce deal. He is demanding the EU agree to major changes to the agreement the bloc made with his predecessor, Theresa May. The EU refuses to renegotiate, so a no-deal Brexit appears increasingly likely.

Many economists say that will trigger a recession and cause economic mayhem, with shortages of fresh food and other goods likely as customs checks snarl Britain’s ports.

Johnson and other Brexit supporters argue that any short-term turbulence will be outweighed by new economic opportunities once Britain leaves the 28-nation bloc and can strike trade deals around the world — notably with the United States. Critics note that the EU accounts for almost half of Britain’s trade and argue that any new trade deals are likely years away.

Hammond criticized the government for perpetuating ‘‘myths’’ that the British people voted for a no-deal Brexit and that leaving the EU without a negotiated settlement would be painless.

‘‘There is no mandate for leaving with no deal,’’ Hammond said. ‘‘It is absurd to suggest that the 52 percent of people that voted to leave the European Union, all voted to leave with no deal when in fact . . . during the referendum campaign there was virtually no mention made by the leaders of that campaign at all of the possibility of leaving with no deal.


‘‘A no-deal exit will cause significant harm to the UK economy and, potentially, irreparable damage to the union of the United Kingdom,’’ he added.

Johnson has refused to rule out suspending Parliament if legislators try to delay or prevent Brexit. Hammond said that would ‘‘provoke a constitutional crisis.’’

House of Commons Speaker John Bercow, who controls the day-to-day business of Parliament, said he would seek to prevent the prime minister from overriding Parliament.

‘‘If there is an attempt to circumvent, to bypass or — God forbid — to close down Parliament, that is anathema to me,’’ Bercow told an audience at the Edinburgh Fringe Festival in comments reported by the Herald newspaper. ‘‘I will fight with every breath in my body to stop that happening.’’

US House Speaker Nancy Pelosi on Wednesday threatened to block any future bilateral trade agreement between the United States and the UK if Brexit puts at risk the accord that brought an end to the conflict in Northern Ireland.

‘‘If Brexit undermines the Good Friday accord, there will be no chance of a US-UK trade agreement passing the Congress,’’ Pelosi said in a statement. ‘‘The peace of the Good Friday Agreement is treasured by the American people and will be fiercely defended on a bicameral and bipartisan basis in the United States Congress.’’


Pelosi’s bid to influence Brexit follows remarks in London by President Trump’s top national security adviser, John Bolton, who said a bilateral trade agreement would be a top priority for the administration after the UK exits the EU and would get ‘‘overwhelming’’ support in the US Congress.

Also on Wednesdy the European Central Bank blasted banks for slow-walking their Brexit preparations, telling them to move additional staff and resources to the European Union in case Britain leaves without a deal on Oct. 31.

The central bank said firms have transferred “significantly fewer activities, critical functions and staff” to their EU operations than originally foreseen, according to a statement on Wednesday. The central bank said some banks are falling short of their supervisory expectations and can’t continue to rely so heavily on servicing EU clients from their branches in the UK.

Material from Bloomberg News was used in this report.