The UK's withdrawal could compromise its access to the bloc's single market if it doesn't maintain EU regulations after Brexit, the European Commission president said on Wednesday.
While speaking at the World Economic Forum in Davos, Ursula von der Leyen cautioned that the "more distance" the UK gets from the European Union's rules, the less access it will have to its market.
"The closer the UK is to the European Union, the better the access to the single market", Ms von der Leyen told the conference.
"If it is the UK's choice not to do so, to be more distant to the European Union, well then there will be more distance to the single market where the level playing field is concerned and where free movement of goods, capital and services is concerned", she added.
She also insisted that trade talks with "our British friends" would begin in February, after rumours surfaced that they would be delayed to the following month.
Von der Leyen's remarks came as Chancellor Sajid Javid pledged to "ditch Brussels rulebook" or there would be "no point" in Brexit.
Mr Javid, who also spoke at Davos, said that a deal could "absolutely" be agreed by December 31.
"There is a strong belief on both sides it can be done", he said.
"Both sides recognise, of course, that it's a tight timetable, a lot needs to be put together in the time that we have but it can be done and it can be done for both goods - where we want to see free trade, zero tariffs, zero quotas - but also on services".
UK Prime Minister Boris Johnson has set a 2021 deadline for the acquisition of a trade deal with the EU, prior the end of the withdrawal agreement process.
If a deal is not secured then then the quo will automatically fall onto World Trade Organisation (WHO) rules and tariffs and quotas will be erected on UK products.
The UK is set to leave the EU on 31 January and will enter into its transition period where it will then have to negotiate a new trade deal with the EU.
Much of the reason for the Brexit vote was spurred by the desire to see the UK not longer be subject to rules set by Brussels.
However, economists have warned that the imposition of tariffs on UK goods could harm domestic industry, putting the British government in a precarious situation.