May's authority on Brexit, already weakened after a failed gamble on a snap election past year, has been further damaged by ideological splits between ministers, exacerbating concerns that Brexit talks may fail and the government collapse.
Brexit-supporting papers have been accused of pushing anti-Semitic memes against Soros.
In response, Soros hit back at what he described as "toxic, personal criticism" and dismissed accusations that his actions were "undermining democracy".
Best for Britain said it would use the money to campaign against Brexit in United Kingdom towns and cities, as well as on social media, to "ensure that stop Brexit - not soft Brexit - is firmly on the table".
The 87-year-old Soros, a refugee from Hungary who made a fortune against the British pound in the early 1990s, says Brexit will be a tragic mistake that will weaken the country's influence.
'The fact that conditions are unsatisfactory does not mean that they can't get worse. "That is what has happened in Britain", he wrote.
"Before the referendum Britain was doing better economically than the rest of Europe". But this has now been reversed, with Continental economies powering ahead while Britain lags behind'. "We, like millions of people, believe that Britain should lead, not leave, Europe", he said.
He added that the problems caused by Brexit would become "painfully obvious" as the date of Britain's exit approached.
The Best for Britain campaign came under fire last week from newspaper articles in the Telegraph and Daily Mail for accepting £400,000 in funding from Soros's Open Society Foundation.
Timothy wrote: 'The objective is to convince MPs to vote against the deal Theresa May negotiates with Brussels, regardless of its content and despite the risk that doing so could mean Britain leaves the European Union with no alternative agreement in place.#.
Conservative lawmaker and prominent critic of May's European Union exit strategy, Anna Soubry, warned on Sunday that the type of Brexit the government was seeking did not have majority support in parliament, which will get a say on the final exit deal.
Michel Barnier, the EU's chief Brexit negotiator, said Friday the transition is "not yet sealed" and that barriers to trade, such as border checks, would be inevitable if Britain opts out of the single market and the customs union.
'This is a democratic and patriotic effort to recover our future and we welcome support for our efforts from many quarters'.