The UK government has warned the EU it is prepared to walk away from trade talks in June unless clear progress is being made in the negotiations.
Whitehall officials said they are ready to step up preparations for leaving the bloc's regulatory regime without an agreement at the end of this year.
The warning came as Downing Street published a policy document setting out what it wants to achieve in the talks, which are due to kick off on Monday.
Under the withdrawal agreement struck with the European Commission by Boris Johnson last year, the UK will continue following most of the EU's rules during a transition period running until the end of this year.
It had been hoped that both sides would be able to strike a free trade agreement before 1 January next year.
But in a sign that the government is prepared to play hardball in the negotiations, the 30-page negotiation mandate makes clear that the PM could call off the talks within three months of them starting.
The document says "the UK is committed to working in a speedy and determined fashion" to make progress between now and a stock-taking meeting in June.
"The government would hope that, by that point, the broad outline of an agreement would be clear and be capable of being rapidly finalised by September," it said.
"If that does not seem to be the case at the June meeting, the government will need to decide whether the UK's attention should move away from negotiations and focus solely on continuing domestic preparations to exit the transition period in an orderly fashion."
Whitehall officials admitted that would lead to customs posts being erected at the UK's border with the EU to allow the checks of goods travelling to and from the continent.
They also made clear that Johnson does not feel bound by the "political declaration" he signed alongside the Brexit withdrawal agreement last autumn setting out the future relationship between the UK and EU.