LONDON — U.K. Labour leader Jeremy Corbyn launched a plea Wednesday, urging fellow opposition parties to back his bid to seize power from Boris Johnson and block a no-deal Brexit, but faced immediate attacks from his would-be allies.

In a letter to party bosses and other senior backbench MPs, Corbyn said he would “seek the confidence of the House [of Commons] for a strictly time-limited temporary government.”

He promised to secure an extension to the Article 50 Brexit process and call an election, in which Labour would campaign for a second referendum with an option of staying in the EU.

But his continued refusal to fully support overturning the 2016 referendum results altogether drew the ire of the party leaders he wrote to.

Prime Minister Johnson has vowed to take the U.K. out of the EU, deal or no deal, by October 31 and has refused to rule out ripping up constitutional norms to do so.

Anti-Brexit parties are reportedly set to meet on Thursday to discuss how to maximize their support across the country.

MPs have been mulling routes to block him, including the option of defeating his administration in a vote of confidence and then forming a cross-party government of national unity.

Corbyn wrote to the Westminster leaders of the Liberal Democrats, the Scottish National Party, Plaid Cymru and the Green Party, which are all supportive of a second EU referendum, urging them to back him as a temporary premier after a vote of no confidence.

He also wrote to Tory backbenchers Dominic Grieve, Oliver Letwin, Nick Boles and Caroline Spelman, who have been plotting to block a no-deal departure.

The Labour leader said their priority “should be to work together in parliament to prevent a deeply damaging no-deal being imposed on the country, denying voters the final say.”

“This government has no mandate for no-deal, and the 2016 EU referendum provided no mandate for no-deal. I therefore intend to table a vote of no confidence at the earliest opportunity when we can be confident of success," Corbyn wrote.

But Lib Dem leader Jo Swinson said Corbyn is "not the person who is going to be able to build an even temporary majority in the House of Commons for this task."

"I would expect there are people in his own party and indeed the necessary Conservative backbenchers who would be unwilling to support him. It is a nonsense," she added.

SNP Westminster leader Ian Blackford said he would work with the Labour leader but said the party "needs to get off the fence on Brexit."

Liz Saville Roberts, the Westminster leader of Plaid Cymru, welcomed the proposal of a national unity government but blasted Corbyn for committing to a general election first over a second Brexit referendum.

“His approach seems to be driven by the fact that Labour know their current frontbench cannot command the confidence of the House of Commons," she said in a statement.

She was echoed by Green MP Caroline Lucas, who said "the proposal from the Labour leader does not guarantee that the people are given the final say on Brexit."

"Holding a general election before a People's Vote is the wrong way around,” Lucas added.

In what appeared to be a pre-emptive response to the appeal from Corbyn, Johnson earlier on Wednesday accused him of wanting to “cancel the referendum and argue about Brexit for years.”

He said on Twitter: “I am committed to leading our country forward and getting Britain out of the EU by October 31.”

A Downing Street spokesman said there is a "clear" choice: "Either Jeremy Corbyn as prime minister, who will overrule the referendum and wreck the economy, or Boris Johnson as prime minister, who will respect the referendum and deliver more money for the NHS and more police on our streets.

"This government believes the people are the masters and votes should be respected, Jeremy Corbyn believes that the people are the servants and politicians can cancel public votes they don’t like."

Anti-Brexit parties are reportedly set to meet on Thursday to discuss how to maximize their support across the country.

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