FILE PHOTO: Britain's Prime Minister Boris Johnson delivers a speech outside Downing Street in London, Britain July 24, 2019. REUTERS/Hannah McKay/File Photo

LONDON (Reuters) - Prime Minister Boris Johnson will tell French President Emmanuel Macron and German Chancellor Angela Merkel that the Westminster parliament cannot stop Brexit and a new deal must be agreed if Britain is to avoid leaving the EU without one.

In his first trip abroad as leader, Johnson is due to meet his European counterparts ahead of a G7 summit on Aug. 24-26 in Biarritz, France.

He will say that Britain is leaving the European Union on Oct. 31, with or without a deal, and that the British parliament cannot block that, according to a Downing Street source.

The United Kingdom is heading towards a constitutional crisis at home and a showdown with the EU as Johnson has repeatedly vowed to leave the bloc on Oct. 31 without a deal unless it agrees to renegotiate the Brexit divorce.

After more than three years of Brexit dominating EU affairs, the bloc has repeatedly refused to reopen the Withdrawal Agreement which includes an Irish border insurance policy that Johnson’s predecessor, Theresa May, agreed in November.

The prime minister is coming under pressure from politicians across the political spectrum to prevent a disorderly departure, with opposition leader Jeremy Corbyn vowing to bring down Johnson’s government in early September to delay Brexit.

It is, however, unclear if lawmakers have the unity or power to use the British parliament to prevent a no-deal Brexit on Oct. 31 - likely to be the United Kingdom’s most significant move since World War Two.

Opponents of no deal say it would be a disaster for what was once one of the West’s most stable democracies. A disorderly divorce, they say, would hurt global growth, send shockwaves through financial markets and weaken London’s claim to be the world’s preeminent financial centre.

Brexit supporters say there may be short-term disruption from a no-deal exit but that the economy will thrive if cut free from what they cast as a doomed experiment in integration that has led to Europe falling behind China and the United States.