STOCKHOLM (Reuters) - A comprehensive agreement on the future relationship between the European Union and Britain will take longer than the 11-month transition period that begins when the UK leaves the EU on Jan. 31, the EU’s chief negotiator said on Thursday.

After Brexit, the two sides will have until the end of the year to negotiate a new trade relationship -- a short period given the complexity of the discussions.

The negotiations are likely to be tough, especially as the EU says that access to the single market will be proportional to how closely Britain sticks to EU rules.

Prime Minister Boris Johnson has said Britain 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,” the EU’s Brexit negotiator Michel Barnier said in a speech in Stockholm.

He said the extremely tight timetable meant the EU would have to prioritise.

First will be building a new capacity to work together with Britain both bilaterally and in global institutions 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,” Barnier said, although he added that cooperation would necessarily be less than currently.

Finally, he said the EU would insist that a future economic partnership is based on a level playing field in regulations, tax and other 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,” Barnier said.

Access to the EU’s vast single market would depend on how tightly the UK aligns itself with the bloc, including in matters such as state aid.

“If the UK wants an open link with us for the products; zero tariffs, zero quota, we need to be careful about what we call zero dumping,” Barnier said.

“If it is not the case ... the level of access for British products will be proportionate.”

Johnson has said London is not seeking a relationship based on alignment with existing rules.

In the longer term, Barnier held out the possibility of an unprecedentedly close relationship with 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,” he said. “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.”