Britain is due to leave the European Union on January 31st

European Commission President Ursula von der Leyen expressed "grave concern" about whether the post-union deal could conclude a trade deal with the UK by the end of 2020.

The UK is due to leave the European Union on January 31st, but will remain in transition until the end of next year as negotiators discuss future trade relations.

"I am very worried given the little time we have," von der Leyen told France's Les Echos newspaper in an interview published Friday. 

"It's not only about negotiating a free trade deal but many other subjects. It seems to me that on both sides we must ask ourselves seriously if all these negotiations are feasible in such a short time," she said.

"I believe that it would be reasonable to review things in the middle of the year, if necessary to see if an extension is needed."

Under the withdrawal agreement, which Prime Minister Boris Johnson negotiated with Europe but has not yet passed in parliament, the United Kingdom may request an extension of one or two years. But Johnson, who won a comfortable majority in the UK parliamentary elections this month, insists that no more time will be needed and is drafting legislation to ban such a move. In this case, the negotiators will only have 11 months to conclude a trade agreement - a task that both sides have warned is extremely ambitious. And if 2020 concludes without a deal, the UK will terminate its links with the EU's vast single market, with no subsequent jobs and trade protection deals on either side.

EU's chief Brexit negotiator Michel Barnier said earlier this week that drafting and ratifying a post-Brexit deal by next year was an "immense challenge but we will give it our all".