Theresa May has been urged to rule out a transitional Brexit deal and ensure the UK's full exit from the EU within two years of negotiations beginning.
Campaign group Leave Means Leave said a "clean, swift" exit should be among the PM's red lines for upcoming talks.
It also said the UK must withdraw from the single market, customs union and common farming and fisheries policies.
On Thursday, a former top EU lawyer warned of a "catastrophe" for the UK if no interim trade accord was struck.
Jean-Claude Piris, head of the EU Council's legal service from 1988 to 2010, said there was no way the UK could negotiate a new free trade deal with the rest of the EU in the two years set aside for determining the UK's exit - warning it would take at least five years and probably more.
He told the Financial Times that the UK must avoid falling into the "WTO gap" - whereby its trading arrangements with the rest of the EU reverted to World Trade Organisation rules with likely tariffs and border checks - and this would require some form of stop-gap agreement.
But Leave Means Leave said there could be no interim arrangement that required the UK to remain in the single market or customs union.
The pressure group, which emerged from the Leave.EU campaign during the EU referendum, said this would be unacceptable as it would require the UK to abide by freedom of movement obligations, remain subject to the European Court of Justice and reduce its scope to strike trade deals outside Europe.
"The UK must leave the EU within two years of triggering Article 50," Richard Tice and John Longworth, the co-chairmen of the organisation, wrote in a letter to the prime minister setting out their principles for the talks ahead.
"There must be no transitional deal on the key issues."
"The EU is renowned for its inability to secure trade deals within a sensible timeframe and the UK must be prepared to walk away and secure trade deals with the rest of the world if the EU fails to agree a deal in this timeframe."
The two men said other "core principles" - including a commitment to end the preferential treatment given to citizens of EU countries in terms of living and working in the UK - should not be sacrificed in the forthcoming talks.
Ministers have suggested a transitional deal remains an option although Mrs May has insisted she believes the outline of a free trade agreement with the rest of the EU can also be resolved during the allotted time.
The prime minister has said she will notify the rest of the EU of the UK's intention to leave - by triggering Article 50 of the 2008 Lisbon Treaty - by the end of March at the latest, paving the way for the UK's potential exit at some point in 2019.