Boris Johnson has formally signed the EU withdrawal agreement, smiling as he described it as a “fantastic moment” for the country.

His enthusiasm came in sharp contrast to the sombre tone of EU officials, who earlier added their signatures to the document before it crossed the Channel.

With a week before the deadline of 31 January, the agreement is now concluded and talks are due to begin over the future relationship between the UK and Brussels.

Johnson said the document’s signing, which took place in Downing Street, would end years of “argument and division” and was a positive change for the UK.

“We can now move forward as one country – with a government focused upon delivering better public services, greater opportunity and unleashing the potential of every corner of our brilliant United Kingdom, while building a strong new relationship with the EU as friends and sovereign equals,” he said.

The pictures of Johnson signing the official agreement after more than two years of negotiations came after a more sober photographic moment for Ursula von der Leyen and Charles Michel, the leaders of the European commission and council respectively.

In images made available of the signing ceremony in the EU’s Europa building on Friday morning, the bloc’s chief negotiator, Michel Barnier, can be seen standing behind the two presidents.

Michel, who as president of the European council will now chair meetings of 27 rather than 28 heads of state and government at the regular summits in the Belgian capital, said in a short statement: “Things will inevitably change but our friendship will remain. We start a new chapter as partners and allies.”

Von der Leyen tweeted: “Charles Michel and I have just signed the agreement on the withdrawal of the UK from the EU, opening the way for its ratification by the European parliament.”

Facebook Twitter Pinterest The EU leaders’ signatures on the withdrawal agreement. Photograph: PA

The signed copy of the agreement was sent in a diplomatic bag to Downing Street for the prime minister’s signature. On Thursday the withdrawal agreement received royal assent from the Queen.

The document, which runs to nearly 600 pages, includes agreements on citizens’ rights, the UK’s £33bn worth of financial obligations to the bloc and the Northern Ireland protocol, establishing the arrangements for maintaining an open border on the island of Ireland.

The European parliament’s constitutional affairs committee backed the agreement on Thursday by 23 votes to three, setting up the final act next week when a plenary session of the EU parliament will vote to ratify the deal.

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Under the agreement, the UK will leave the EU at midnight central European time on 31 January. The UK will remain in the EU’s single market and customs union, but none of the decision-making bodies, until the end of 2020.

Facebook Twitter Pinterest Ursula von der Leyen signs the agreement. Photograph: PA

Johnson has said he will not take the option available to him in the withdrawal agreement of extending this transition period. The prime minister has said the 11-month period available without further extension is “ample” time to reach agreement on a comprehensive deal.

The EU’s negotiating position, known as a mandate, is due to be adopted on 25 February by EU ministers attending a general affairs council, and formal negotiations will then begin on the future relationship.

A political declaration on the outlines of the future relationship, which has already been given the political seal of approval in London and the EU capitals, will set the parameters of the talks as the two negotiating teams seek to replace the terms of the UK’s 45-year membership with new arrangements.