One of the co-founders of the fashion label Superdry has donated £1m to the campaign for another EU referendum.
Multi-millionaire Julian Dunkerton said he was backing the People's Vote campaign because "we have a genuine chance to turn this around".
The People's Vote, a cross-party group including some MPs, want a public vote on the final Brexit deal.
The government has ruled out another referendum after Britain voted to leave the EU in June 2016.
The UK is on course to leave the EU on 29 March next year.
Mr Dunkerton said he believes the brand he co-launched "would never have become the global success that it did" if Brexit had happened 20 years earlier.
His donation, the largest received by the People's Vote, will go towards funding opinion polls.
He added: "I will be paying for one of the most detailed polling exercises ever undertaken by a campaign so that more and more people have the confidence to demand the democratic right for their voice to be heard."
However, Conservative MP Andrew Bridgen - a leading Brexit backer - said the People's Vote campaign was not about democracy.
He told the BBC: "Actually what their declared intent is is to overturn the democratic decision of 2016 to leave the EU."
The former UKIP leader, Nigel Farage, said that he doubted another public vote would reverse the referendum result.
He told his LBC radio show he "had a hunch" that if the public got the chance to vote again "then our politicians might just be in for the shock of their lives".
His comments come a day after after he announced he would return to the campaign trail and offer his "total support" to pro-Brexit group Leave Means Leave.
The People's Vote campaign is hoping to get enough MPs, including the Labour leadership, to back a referendum on the deal Prime Minister Theresa May strikes with Brussels, which is due to be put to a vote in Parliament in October.
They have organised a number of events over the summer to step up pressure on MPs.
Hundreds of people attended a rally in Edinburgh's Festival Square on Saturday in support of the campaign.
Bank chief warning
Meanwhile, a Standard Chartered bank executive says tough demands by EU regulators could see more jobs in the industry being moved overseas from the UK than currently thought.
Standard Chartered is to spend about £15m turning its Frankfurt office into a European base due to Brexit.
Europe and Americas boss Tracy Clarke says the impact on banks with large EU services may be "significant".
On Thursday the government is set to publish the first in a series of technical notices designed to prepare the UK for the possibility of a no-deal Brexit.
Brexit Secretary Dominic Raab said securing a deal was still "the most likely outcome" - but added making alternative arrangements was the "responsible" thing to do.
Between late August and the end of September, government departments are expected to publish around 70 technical notices.