Theresa May is being urged to walk away from Brexit negotiations this week if EU leaders refuse to start trade talks.
The call comes from a group of pro-Brexit Tory and Labour politicians, including former Chancellor Lord Lawson, as well as business leaders.
The prime minister is to push for the deadlocked talks to move to the next phase at a meeting of EU leaders in Brussels on Thursday.
But EU officials are not expecting any movement at the summit.
The other 27 leaders are expected to deliver a verdict on progress on Friday that will say Britain must make an offer on the so-called divorce bill before they will talk about trade, according to officials quoted by Reuters.
But European Council President Donald Tusk is expected to propose to the 27 EU leaders that they begin talks amongst themselves about Britain's future relationship with the EU, when it leaves the bloc in March 2019.
In a letter to Theresa May, organised by the Leave Means Leave campaign, the leading Brexiteers say the government "has been more than patient" towards the EU and "decisive action" was now needed to end the "highly damaging" levels of uncertainty facing businesses.
In the event of no progress at Thursday's European Council meeting, the letter says, Mrs May should formally declare that the UK is working on the assumption that it will be reverting to World Trade Organisation (WTO) rules on 30 March, 2019.
Early notification of such a move would allow the UK to "concentrate our resources on resolving administrative issues" and prepare to "crystallise the economic opportunities" of Brexit, it adds.
The letter is signed by Lord Lawson, Conservative former ministers Owen Paterson and Peter Lilley, Labour MPs Kate Hoey, Graham Stringer and Kelvin Hopkins, Wetherspoons pub boss Tim Martin and home shopping magnate and Labour donor John Mills, as well as pro-Brexit academics and former military figures.
The UK government says it does want to leave the EU without a deal in place and is targeting a new free trade arrangement to replace its current single market membership.
But speaking on BBC Radio 4's Today programme, Mr Paterson said it appeared unlikely that a trade deal would be struck with the EU "because they are flatly refusing to talk about it".
Instead, he said, there was a "complete obsession with money" - the amount the UK is required to pay as it leaves the EU.
It was "inevitable at the moment, it is an ineluctable certainty we are going to end up with WTO at the end of this anyway" so it was better to "state that now" and give business time to prepare, he added.
But Labour's Brexit spokesman Sir Keir Starmer said: "This is the first phase of the first set of negotiations and talking of walking away at this stage is irresponsible."