The anti-Brexit group backed by George Soros is set to publish its "manifesto" for keeping Britain in the EU on June 8 — exactly one year on from the last general election.

Best For Britain will publish a " roadmap to stopping Brexit" with a referendum in the next year, insiders tell Business Insider.

The campaign — first set up by banker turned political activist Gina Miller — will also say that a soft Brexit would be a "disaster".

Best For Britain has received millions of pounds worth of donations.

LONDON — Best For Britain — the anti-Brexit campaign backed by George Soros — will publish its official plan for reversing Britain's exit from the European Union on the one year anniversary of last year's UK general election.

The plan, which is set to be launched next Friday (June 8), will be a "roadmap to stopping Brexit" including a case for "why a soft Brexit is a disaster," a senior group insider has told Business Insider.

Soros, a global investor and philanthropist, said in a speech in Paris on Monday that the campaign for a second referendum would begin "in the next few days" and that there was time left for Britain to "rescind" Brexit.

"Best for Britain fought for, and helped to win, a meaningful parliamentary vote which includes the option of not leaving at all. This would be good for Britain but would also render Europe a great service by rescinding Brexit and not creating a hard-to-fill hole in the European budget," Soros told the European Council on Foreign Relations think tank.

"But the British public must express its support by a convincing margin in order to be taken seriously by Europe. That’s what Best for Britain is aiming for by engaging the electorate. It will publish its manifesto in the next few days."

Soros did not reveal an exact launch date for the plan, or what it would include.

However, BI understands that it will be published on June 8. This is exactly one year one from the date of the 2017 general election in which Theresa May lost her majority and arguably her mandate for a hard Brexit.

The "roadmap" is to explain there can be another referendum on Brexit within the next twelve months which the Remain campaign can with a "convincing margin" in order to keep Britain in the EU.

It will also lay out a case against the softer form of Brexit currently being pushed by Remain-backing MPs in the House of Commons. MPs in the Conservatives, Labour and other opposition parties are planning to vote for amendments to the EU (Withdrawal) Bill which seek to keep Britain in a customs union with EU and inside the single market.

However, Best For Britain will use its "manifesto" to distance itself from the soft-Brexit movement, amid increasing tension between a number of pro-EU MPs and campaign groups who call for Brexit to be cancelled altogether, including Best For Britain and the Chuka Umunna-backed Open Britain.

A number of Labour MPs told the Huff Post that MPs who support the aforementioned groups have failed to take their campaigns to Leave-supporting areas amid rising tensions about the direction of the pro-EU movement in the UK.

Speaking about Open Britain's campaign to persuade MPs to back a Norway-style Brexit, one shadow frontbencher told BI: "They can't say they want to stop Brexit and then also say what sort of Brexit we should have."

Soros has already donated at least £700,00 to Best For Britain, which was first set up last year by Gina Miller, the banker turned political activist who in 2016 successfully took the government to court over Brexit. The group has received millions of pounds worth of donations from a mixture of grassroots campaigners and high-profile political and business figures.