Brexit group Vote Leave has been designated the official Leave campaign for the upcoming EU referendum.
The Electoral Commission has today ruled the group, which has the support of London Mayor Boris Johnson, former Tory leader Iain Duncan Smith and four Cabinet Ministers including Michael Gove, will be the lead group for the June 23 vote.
The decision means the rival Grassroots Out group, which was backed by businessman Arron Banks, Ukip leader Nigel Farage and Tory backbenchers Peter Bone and Tom Pursglove, have missed out.
A spokesman for Banks told The Huffington Post he was seeking legal advice this afternoon on trying to get the decision overturned.
Andy Wigmore said: "We are going to appeal it, which could delay the referendum until October 23.
"We think it's a political stitch-up."
Banks will announce his decision on whether to legally appeal against the decision by 12noon on Thursday.
Ukip leader Nigel Farage, who could now face a diminished role in the campaign, congratulated Vote Leave on getting designation.
His spokesman said the MEP did not support Banks' decision to seek a judicial review of the designation, and said: "The die is cast and Outers should get on with it."
In a statement, Farage said: "The decision of UKIP to back Grassroots Out was on the basis that they saw the importance of the immigration issue in this referendum, and that we wanted to reach out across the country at a grassroots level and work with everyone from the left, centre and right of British politics to get our country out of the EU. I believe this approach is the only way the Leave side can win this referendum.
"It is clear that Vote Leave now share my view on this approach, for instance the issue of EU open borders is now a prominent part of their campaign messaging.
"Regardless of whichever campaign got the designation, UKIP would always have played a big role in this campaign as the only national party committed to leaving the EU and with a substantial £4 million spending limit.
"I have always wanted all on the Leave side to come together and have done my best to try and make this happen. I'll continue to do so in the run up to the referendum to ensure the Leave side wins.
"We in UKIP, as I've said from the start, will work with anyone that wants to leave the EU. We must work together to get our country out of the European Union."
The decision means Vote Leave will receive public money to fund its campaign, and will have a spending limit of £7million.
Non-designated groups will only be allowed to spend £700,000.
Vote Leave will also be entitled to one mass mail-out to voters, similar to the controversial pro-EU leaflet the Government sent out this week.
Ukip MEP Patrick O'Flynn urged Brexiters from all camps to join together for the battle ahead.
He told the Huff Post UK: "It's been a hotly contested process, but now the designation has been made and we have 70 days to convince the British public to bring home their democracy.
"Now is the time for all Ukip members and Better Off Outers in general to get behind the campaign and focus on taking on our real enemy - those who want to keep Britain ruled by Brussels."
Explaining the decision, Claire Bassett, Chief Executive of the Electoral Commission, suggested Vote Leave was able to represent all campaigners trying to get the public to back Brexit in the referendum.
She said: "After careful consideration, the Commission decided that ‘Vote Leave Ltd’ better demonstrated that it has the structures in place to ensure the views of other campaigners are represented in the delivery of its campaign. It therefore represents, to a greater extent than ‘Go Movement Ltd’, those campaigning for the ‘Leave’ outcome, which is the test we must apply.
"We encourage all campaigners to now focus on engaging voters on the historic decision they will have to take on 23 June."
Ukip MP Douglas Carswell backed Vote Leave for designation despite his party leader supporting the rival campaign.
That prompted a split between the Clacton MP and Farage and Banks - who bankrolled Grassroots Out and another Brexit group Leave.EU.
Bone, who co-founded Grassroots Out, said: "We congratulate Vote Leave on securing designation and we thank our supporters for all the hard work they have put into the campaign so far.
"We draw our support from right across the political spectrum and from tens of thousands of grassroots campaigners of no particular political affiliation.
"We are motivated by a deep sense of patriotism and the unflinching belief that the UK will be safer, stronger and more independent outside the EU. We will be able to control our own borders, build our prosperity by trading freely with the whole world, and restore our historic right to self government.
"We look forward to working closely and productively with all those who want to see the UK set free to determine its own destiny. We are determined to play our part in creating a united front to secure victory on June 23 for Leave – Independence Day."