Vote Leave and Britain Stronger in Europe have been designated as the official Leave and Remain campaigns in June's EU referendum.
Vote Leave - backed by Boris Johnson and Michael Gove - saw off a challenge from a rival campaign Grassroots Out, backed by UKIP leader Nigel Farage.
The campaigns will be allowed to spend up to £7m, get a free mailshot, TV broadcasts and £600,000 public funds.
The Electoral Commission made the decision after weeks of deliberation.
It is likely to have a major impact on the shape of the debate in the run up to 23 June's referendum.
It means Nigel Farage will not feature on official Leave campaign literature, with Vote Leave insiders fearing the UKIP leader would alienate centre ground voters they believe hold the key to winning. The group wants to make a "positive" case for leaving the EU, with less emphasis on immigration.
Britain Stronger in Europe is likely to continue to focus on the impact of an exit on jobs and economic prosperity.
Grassroots Out founder, Tory MP Peter Bone, said his campaign would continue, but with a spending limit of £700,000, as he attempted to draw a line under the bitter war of words between his group and Vote Leave.
He added: "We look forward to working closely and productively with all those who want to see the UK set free to determine its own destiny."
A spokesman for Vote Leave said: "Our focus has always been the real campaign and the £350m we send to Brussels every week which we want to spend on our priorities like the NHS. We will continue to work constructively with everyone who wants to campaign for a Leave vote."
Nigel Farage offered an olive branch to Vote Leave, saying he could work with them as they had accepted his argument that immigration must play a more central role in the debate.
The UKIP leader added: "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 £4m 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."
But Leave.EU, a group backed by UKIP donor Arron Banks, which supported Grassroots Out, said it was planning to seek a judicial review of the Electoral Commission's decision, with Mr Banks claiming it "smells of political corruption" and did not make sense.
'Unity of purpose'
The Trade Union and Socialist Coalition also applied for designation as the official Leave campaign, claiming left-wing anti-EU views were not being properly represented, but its application was also rejected by the Electoral Commission.
Meanwhile, Prime Minister David Cameron - who is backing a vote to Remain in the EU - welcomed the designation of Britain Stronger in Europe with a message on Twitter: "Congratulations to @StrongerIn who have been designated as the Remain campaign in the EU referendum. We're stronger, safer and better off in."
Britain Stronger in Europe - chaired by former Marks and Spencer boss Stuart, now Lord, Rose - was the only campaign to apply for Remain designation.
The campaign's executive director Will Straw said: "I'm delighted that the Electoral Commission have recognised that Britain Stronger in Europe has the breadth of support, the unity of purpose and the campaigning organisation to be the official Remain campaign in the upcoming referendum."
He congratulated Vote Leave on gaining the designation but called on them to "come clean with the British people and say what Out looks like".
The elections watchdog said it had based its decision on the campaign "which appears to us to represent those campaigning for that outcome to the greatest extent".
The Commission's assessment scorecard, which took into account factors such as breadth of support and funding levels, gave the Out designation to Vote Leave by 49 to 45.
Electoral Commission chief executive Claire Bassett 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."