* Sterling volatility gauges jump
* Opposition Labour Party to seek emergency debate
* Johnson heading for an election - lawmakers
* EU ministers warn against no-deal Brexit (Adds statement from opposition parties in paragraph 8)
By William James and Guy Faulconbridge
LONDON, Aug 29 (Reuters) - The opposition Labour Party said on Thursday it would trigger an emergency debate in parliament next week to try to stop Prime Minister Boris Johnson taking Britain out of the European Union without a withdrawal deal.
More than three years after the country voted in a referendum to leave the bloc, the United Kingdom is heading towards its gravest constitutional crisis in decades and a showdown with the EU over Brexit, which is due to take place in just over two months time.
Johnson, who became prime minister last month, enraged opponents of a no-deal Brexit on Wednesday by using a parliamentary mechanism to order the suspension of parliament for almost a month.
The speaker of the lower house of parliament, John Bercow, called this a constitutional outrage as it limited the time the parliament has to debate and shape the course of British history.
Labour Party leader Jeremy Corbyn said that as soon as parliament returns from its summer break on Tuesday, his party would initiate a process to legislate against a no-deal Brexit that he said would be damaging for the jobs and the economy.
"What we are going to do is try to politically stop him (Johnson) on Tuesday with a parliamentary process in order to legislate to prevent a no-deal Brexit and also to try and prevent him shutting down parliament in this utterly crucial period," Corbyn told reporters.
"This country is in danger of crashing out on the 31st of October with no deal," he said. "We have got to stop that and that is exactly what we will be doing next Tuesday."
Five other opposition parties, including the Liberal Democrats and the Scottish National Party, later issued a joint statement with Labour calling on Johnson to let legislators vote on whether parliament should be suspended.
Johnson's move to suspend parliament for longer than usual was cheered by U.S. President Donald Trump but provoked strong criticism from some British lawmakers, including some of Johnson's own ruling Conservative Party, and media.
BREXIT CRISIS
After years of tortuous negotiations and a series of political crises since the United Kingdom voted 52% to 48% to leave the EU in the 2016 referendum, Brexit remains up in the air. Options range from an acrimonious divorce on Oct. 31 and an election to an amicable exit or even another referendum.