Nigel Farage has "bottled it" after opting to stand down Brexit Party candidates in almost half the available seats, according to pro-EU figures.
The Brexit Party leader announced on Monday that his candidates would not contest the 317 seats won by the Conservatives at the 2017 election but has vowed to go after every Labour held seat.
Labour leader Jeremy Corbyn said the "pact" between Mr Farage and Boris Johnson was a "Trump alliance".
"One week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson," he tweeted.
"Today, Trump got his wish. This Trump alliance is Thatcherism on steroids and could send £500 million a week from our NHS to big drugs companies. It must be stopped."
One week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson.
Today, Trump got his wish.
This Trump alliance is Thatcherism on steroids and could send £500 million a week from our NHS to big drugs companies. It must be stopped.
— Jeremy Corbyn (@jeremycorbyn) November 11, 2019
Naomi Smith, the chief executive of pro-EU campaign group Best for Britain, accused the former Ukip leader of having "bottled it" and intensified calls for Remainers to vote tactically.
"Farage has bottled it and hung most of his own candidates out to dry," she said.
"But by standing down Brexit Party candidates across the country, it's now more important than ever that Remainers use their votes wisely.
"Our best chance of stopping a nightmarish government delivering a hard and damaging Brexit is voting tactically."
Remain-supporting parties were united in their condemnation of the move, a shift that is likely to boost the Tories in areas such as the South West where Mr Farage said he feared splitting the vote and aiding the Liberal Democrats to win seats.
Lib Dem leader Jo Swinson hit back, saying: "The Conservative Party are the Brexit Party now."
Fellow pro-EU leader, the SNP's Nicola Sturgeon, tweeted: "Any form of Brexit that is acceptable to Nigel Farage will be deeply damaging for Scotland.
"Makes it all the more important to get rid of Boris Johnson's Tories, escape Brexit and put Scotland's future into Scotland's hands."
The Tory party has now effectively become the Brexit party. @theSNP is the main challenger in all of their Scottish seats. Defeating the Tories here will help deprive Boris Johnson's increasingly extreme and right wing party of the majority they crave.
— Nicola Sturgeon (@NicolaSturgeon) November 11, 2019
Caroline Lucas, the former Green Party leader, said Mr Farage's decision was evidence that the "Conservatives have morphed into the same narrow sect as Brexit Party".
"This is the alliance Trump wanted. We must resist delivering Trump the result he wants too," said Ms Lucas.
Plaid Cymru leader Adam Price suggested the fresh alliance was "proof" that the Prime Minister and Mr Farage were jointly "planning to deliver a disastrous no deal".
The news of the Brexit Party making way for Tory candidates was welcomed by Mr Johnson, who told reporters: "I'm glad that there's a recognition that there's only one way to get Brexit done and that's to vote for the Conservatives."
Members of Mr Farage's own party have been accepting of their leader's decision.
Christopher Kenny watched the speech in Hartlepool and said he was "pleased" with the party's position.
"I'm pretty pleased he has done it," said the 39-year-old.
"We want out and that seems to be the best way of going about it, as long as Boris can keep his word."