Guardian | EU looks at extending Brexit transition period beyond 2020

At the end of the transition period on 31 December 2020, the UK is set to exit the EU's customs union and single market and enter newly negotiated arrangements. Ursula von der Leyen, the European commission's president, said that negotiating a future relationship in such a short time frame would be very challenging.
[...]
The withdrawal agreement stipulates that the transition can be extended by "one or two years" but that this must be agreed before 1 July next year. It is recognised in Brussels that Johnson will find it politically impossible to seek an extension to the transition to allow all the issues to be agreed.

On breaking his word by asking for a delay, Johnson would have to open negotiations on how much extra the UK would pay into the EU budget. Free movement of people would also continue.

Instead, it is understood initial discussions have taken place in Brussels about the EU asking the British government for an extension, given the complexity of the talks. It is likely that the EU would also need to sweeten the offer of an extension by minimising the costs that the UK would face.

euractiv | EU leaders congratulate Johnson but warn of more hard work ahead

EU leaders were set to charge EU negotiator Michel Barnier on Friday with negotiating a future trade deal with Britain. They were also set to discuss the future relationship with the UK.

"We expect a vote on the withdrawal agreement as soon as possible. The EU is ready for the next phase. We will negotiate a future trade deal which ensures a true level playing field," said the European Council President Charles Michel.

Varadker

"What I'd like to see is a new future economic partnership, a trade deal if you like, or a trade deal plus, between the UK and the EU, including Ireland," he said and stressed the key was to ensure "a tariff-free trade between the UK and the EU, as well as a set of minimum standards".

Weber

"Looking forward, we need the UK as a strong partner, but being outside the EU means losing benefits. We share the ambition to move quickly, but let's be clear: a future trade deal will be even more complex than the Withdrawal Agreement. Let the real work begin," he tweeted.

Leaders ask the Commission for a mandate to start post-Brexit negotiations with UK
"'Chapeau that he was able to pull this off,' German Chancellor Angela Merkel told a press conference following the 12-13 December summit."