Britain's post-Brexit trade deal with the European Union provides "a new stability and a new certainty", Prime Minister Boris Johnson said Thursday, adding UK-EU relations had sometimes been "fractious and difficult".

"We will be your friend, your ally, your supporter and indeed never let it be forgotten your number one market," he said at a press conference announcing the agreement.

He said: "Tonight, on Christmas Eve, I have a small present for anyone who may be looking for something to read in that sleepy post-Christmas lunch moment, and here it is, tidings, glad tidings of great joy because this is a deal.

"A deal to give certainty to business, travellers, and all investors in our country from January 1. A deal with our friends and partners in the EU."

Earlier, at a Downing Street press conference soon after the deal was announced, Mr Johnson said: "We have taken back control of our laws and our destiny," but stressed the UK "will remain culturally, emotionally, historically, strategically and geologically attached to Europe".

In a press conference in Brussels, European Commission President, Ursula von der Leyen said the deal was "fair" and "balanced" and it was now "time to turn the page and look to the future". The UK "remains a trusted partner," she added.

At his press conference, Boris Johnson said the £668bn a year agreement would "protect jobs across this country".