Billionaire investor George Soros is to pump a further £100,000 into the anti-Brexit campaign.
The move comes after Mr Soros drew fire from Brexiteers when it emerged he was donating £400,000 to the campaign group Best for Britain to try and halt withdrawal from the EU.
The group has now revealed Mr Soros's OFS Foundation has also pledged to match crowd-funding to it of up to £100,000.
The group's CEO Eloise Todd said Brexit could still be stopped by a meaningful vote in Parliament.
"The UK's future with the EU is not a done deal, there is still a vote to come and people across the country deserve to know the truth about the options on the table: one of which is staying and leading in the EU."
Hungarian-born Mr Soros, one of the world's richest men who made a billion dollars betting against sterling on Black Wednesday in 1992, accused critics of a maligning him, telling The Guardian: "I am happy to take the fight to those who have tried to use a smear campaign, not arguments, to prop up their failing case."
The move followed Justice Secretary David Gauke insisting there is no "plot to gag" Cabinet members who want a softer Brexit.
The remarks came after it emerged the three leading Cabinet Brexiteers will all give keynote speeches on the UK's EU withdrawal stance, but Chancellor Philip Hammond will not take part in the co-ordinated bid to set out the Government's position.
Mr Gauke told ITV's Peston on Sunday: "He is not part of the set of speeches that have been outlined today, but that doesn't mean that the Chancellor is not expressing his views both internally in the Cabinet conversations, but also externally.
"So, I don't think that there really is anything in this, that this is somehow any kind of plot to gag a particular faction of ministers. I don't think that's a fair characterisation at all."
In a bid to regain the initiative on Brexit after a rocky few weeks during which Cabinet tensions came to the surface, Prime Minister Theresa May is set to make two keynote addresses.
Foreign Secretary Boris Johnson, Brexit Secretary David Davis, and International Trade Secretary Liam Fox will also deliver speeches, but the only minister who backed Remain taking part in the project is Cabinet Office Minister David Lidington.
Mr Johnson will kick-off the Brexit blitz on Wednesday, Valentine's Day, with a call for unity over Brexit.
Mrs May will deliver a major speech on post-Brexit UK-EU security in Germany next Saturday, and will round off the process in about three weeks' time with a keynote address on the overall relationship, following a special "away day" summit of the Cabinet withdrawal committee at Chequers.
However, Tory tensions remained to the fore as former minister and leading rebel Anna Soubry delivered a warning to the PM.
Asked if she believed there is a majority in the House of Commons to defeat "the kind of Brexit the Prime Minister wants", Ms Soubry told BBC1's The Andrew Marr Show: "If she's not careful, yes."
Pressed on whether she thought Brexit will definitely happen, Ms Soubry, who appeared on the programme with pro-Europe Labour MP Chuka Umunna, said: "Will it definitely happen? I genuinely don't know what is going to happen."
When Mr Marr suggested that Ms Soubry is closer in her politics to Mr Umunna than she is to Jacob Rees-Mogg, she said: "I'm not denying that."
International Development Secretary Penny Mordaunt said the series of withdrawal speeches would set out a Brexit "vision".
She told The Andrew Marr Show: "What the public want is, they want the vision and they want some meat on the bones."
Ms Mordaunt insisted she believed a transition period was "a given" despite claims to the contrary by EU chief negotiator Michel Barnier.
She said: "What I would say to the public is that, actually, the other nations involved in this are very pragmatic and have not been impressed with some of the language that the (European) Commission has used."
Asked if it was a Government red line to not have to grant full rights to EU migrants who come to Britain during a transition period, Ms Mordaunt said: "It is what we are setting out in our position. Again, all of this is a negotiation."