A comprehensive agreement on the future relationship between the European Union and Britain will take longer to agree than the 11-month transition period that begins when the UK leaves the EU at the end of January, the EU's chief negotiator has said.

Britain is due to leave the bloc on 31 January and the sides will then have until the end of the year to negotiate a new trade relationship.

British Prime Minister Boris Johnson has said the UK will not extend the transition period beyond December.

"We are ready to do our best and to do the maximum in the 11 months to secure a basic agreement with the UK, but we will need more time to agree on each and every point of this political declaration," Michel Barnier said in a speech in Stockholm.

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Mr Barnier said that given the extremely tight timetable, the EU would have to prioritise over the next months.

First will be building a new capacity to work together with Britain both bilaterally and in global institution to address issues such as climate change and peace in the Middle East.

"The second point is that we need to build a very close security relationship," Mr Barnier said.

However, he said that a future security relationship with the UK outside the EU would mean that the level of cooperation would necessarily be less than currently.

Finally, he said that the EU would insist on a future economic partnership based on a level playing field in terms of social, environmental and tax matters.

"Competing on social and environmental standards can only lead to a race to the bottom that puts workers, consumers and the planet on the losing side," Mr Barnier said.

Mr Johnson has said the UK is not seeking a new relationship based on alignment with existing rules.

In the longer term, Mr Barnier held out the possibility of an unprecedentedly close relationship between Europe and Britain.

"We will strive for a partnership that goes well beyond trade ... covering everything from services and fisheries to climate action energy transport, space, security and defence, but that is a very huge agenda and we simply cannot expect to agree on every single aspect of this new partnership in under one year," he said