Britain is heading into a new chapter in its relationship with the rest of Europe with Boris Johnson vowing to pit the country against the EU in a race for economic success. After nine months of tortuous talks, a Brexit deal was secured at 1.44pm GMT on Christmas Eve, avoiding a no-deal exit from the transition period with just a week to go. The deal was met with “relief tinged with some sadness” in Brussels. But it was said by Johnson to be the settlement of the vexed question of Britain’s relationship with Europe, 48 years since the country joined the then European economic community. Flanked by large union flags, the prime minister said: “This European question’s been going on for decades. I think this gives us the platform, the foundation for a really prosperous new relationship.” The deal delivered on the promise of a “giant free-trade zone” characterised by “regulatory competition”, Johnson said. “We have taken back control of laws and our destiny …We have taken back control of every jot and tittle of our regulation in a way that is complete and unfettered.” The prime minister said the UK had won the right “set our own standards, to innovate in the way that we want” in key sectors such as biosciences and artificial intelligence. “British laws will be made solely by the British parliament; interpreted by British judges, sitting in the UK courts,” he said. He later tweeted a video where he hailed the deal as “great tidings of great joy”. Picking up a copy of the 500-page document, he added: “The oven-ready deal was just the starter. This is the feast – full of fish, by the way. I believe it will be the basis of a happy and successful and stable partnership with our friends in the EU for years to come.” His optimism contrasted with the expectations of the government’s own independent forecaster, the Office for Budget Responsibility, which expects Brexit to shave 4% off GDP in the medium term. From 1 January, most UK nationals will lose the right of free movement and British businesses will face significant extra costs in doing business with its biggest export market. The European commission president, Ursula von der Leyen, gently questioned Johnson’s understanding of sovereignty, the watchword of the British negotiation led by David Frost, as she welcomed the agreement. She said: “Of course, this whole debate has always been about sovereignty. But we should cut through the sound bites and ask ourselves what sovereignty actually means in the 21st century. “For me, it is about being able to seamlessly do work, travel, study and do business in 27 countries. It is about pooling our strength and speaking together in a world full of great powers. And in a time of crisis, it is about pulling each other up. Instead of trying to get back to your feet, alone. “And the European Union shows how this works in practice. No deal in the world can change the reality of [the] gravity in today’s economy. And in today’s world, we are one of the giants.” Von der Leyen referenced Shakespeare, the Beatles and TS Eliot as she ushered in the new era. “It was a long and winding road, but we have got a good deal to show for it. It is fair. It is a balanced deal. And it is the right and responsible thing to do for both sides,” she said. “At the end of a successful negotiation I normally feel joy. But today I only feel quiet satisfaction and, frankly speaking, relief. “I know this is a difficult day for some, and to our friends in the United Kingdom, I want to say parting is such sweet sorrow but, to use the line from TS Eliot, what we call the beginning is often the end. And to make an end is often a beginning. So, to all Europeans, I think it is time to leave Brexit behind.” Most of Johnson’s themes in his speech, including sovereignty, regulatory independence and fishing, were those close to the hearts of the Brexit-backing MPs who helped him to win the Tory leadership last year. With MPs set to vote on the agreement on Wednesday 30 December, Downing Street has kept Eurosceptic backbenchers closely informed as negotiations developed. After the deal was struck, many said they would await the verdict of a “star chamber” of experts convened by the backbench European Research Group to go through the agreement line by line. The group, consisting of lawyers and politicians including the former Europe minister David Jones, is expected to publish its conclusions on Monday. In a sign of battles to come, representatives of the fishing industry said it was “bitterly disappointed” with what Johnson described as a “prodigious” increase in the amount of fish the UK could land. Barrie Deas, head of the National Federation of Fishermen’s Organisations, said Johnson had had his “Ted Heath moment” – a reference to his predecessor giving away fishing rights in British waters in 1973. A transition period of five-and-a-half years to phase in changes, with the UK accepting a 25% repatriation of quotas, amounted to “justice deferred, justice denied”, he said. The prime minister urged Brexiters and remainers to move on from the divisions that have plagued UK politics since the referendum. “My message to everybody on both sides of that argument in 2016 is I really think now it’s a long time behind us,” he said. He claimed the UK would remain “culturally, emotionally, historically, strategically and geologically attached to Europe, not least through the 4 million EU nationals who have requested to settle in the UK over the last four years and who make an enormous contribution to our country and to our lives”. The EU’s chief negotiator, Michel Barnier, said it was “a day of relief but tinged with some sadness”. He lamented the government’s decision not to be a partner in the Erasmus student exchange programme and the lack of cooperation on foreign policy and defence. The deal will be scrutinised by EU leaders but both the French president, Emmanuel Macron, and Angela Merkel, the German chancellor, said they believed the accord would be in place for the end of the year.