Britain could be fined millions of pounds if it tries to negotiate trade agreements with other countries before leaving the European Union.
The European Commission and other EU countries could take the UK to court if the Government begins talks with countries already in negotiations with the EU.
Failing to comply with demands from Brussels could lead to “infraction proceedings brought by the European Commission” and “infringement actions by member states”, papers seen by The Sunday Times suggest.
If such legal proceedings were brought against the UK, it would be “required to pay a fine”.
A Government spokesperson told The Independent: “We are not going to provide a running commentary on leaving the EU. The role of the Department for International Trade is to explore trading opportunities following Brexit and we will get the best possible deal for the whole of the UK as we forge a new global role for ourselves in the world.
“As we have said, we will fully comply with our responsibilities and exercise our rights as a member of the EU.”
Donald Tusk, President of the European Council has said Prime Minister Theresa May told him during talks at No 10 last week it was “quite likely” she would be ready to trigger Article 50 of the Lisbon Treaty in January or February next year.
However, a Downing Street source told The Independent it was an “interpretation” of their conversation and Ms May did not specifically mention January or February.
Speaking about his meeting with Ms May, Mr Tusk told a press conference on Friday: “Prime Minister May was very open and honest with me.
“She declared that it's almost impossible to trigger Article 50 this year but it's quite likely that they will be ready maybe in January, maybe in February next year.”
Speaking at an informal EU summit in the Slovakian capital, Bratislava, European Commission President Jean-Claude Juncker insisted Britain cannot get access to the European single market without accepting the free movement of workers.
Mr Juncker said: “There’s a clear interlink as we made clear since the very beginning between the access to the internal market and the basic principles of the internal market, mainly the one of the freedom of movement of workers.
“We are sticking to that position and this is not a game between prime ministers leaving and prime ministers remaining, this is about people in Europe.
“It's about the rights of ordinary people and workers, of those living in Europe, and so I can’t see any possibility of compromising on that very issue.”
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