LONDON, June 24 (Xinhua) -- British Prime Minister Theresa May was told Sunday by 60 influential politicians, economists and business leaders she must prepare for a no-deal Brexit.

The 60 former senior government ministers, MPs, economists and business figures have sent a jointly signed letter to 10 Downing Street in which they call on May to issue orders to government departments to accelerate planning for Britain to operate outside the European Union under World Trade Organization (WTO) rules if a deal with Brussels cannot be done.

The call comes just 24 hours after 100,000 people marched through London calling for a people's vote on a Brexit deal.

The latest letter, organised by Economists for Free Trade (EFT), has been signed by former chancellor Lord Lawson, ex-environment secretary as well as Tim Martin, pre-leave head of the British bar and restaurant chain Wetherspoons who recently ordered his bars not to sell European beers.

The letter calls on May to tell the EU's Brexit negotiators that her preferred outcome is a free trade deal between Britain and the EU,

But it adds: "However, in light of the reluctance of the EU swiftly to secure a free trade deal we suggest you make clear your belief that the UK has now to prepare urgently for the possibility that no agreement is forthcoming."

They say they believe now is the time to accelerate preparations for no deal and a move to a World Trade Deal under WTO rules.

They also say Britain must reserve the right to walk away without a trade deal and take with it the divorce package it has offered worth around 52 billion U.S. dollars.

May has not responded directly to the letter, but a spokesperson at her Downing Street office said: "We are confident of getting a good deal that delivers for every part of the UK and allows us to take back control of our money, rules and borders."

Meanwhile, Health Secretary Jeremy Hunt became the first senior member of May's front-bench cabinet to respond to concerns about Brexit raised by Airbus and the car maker BMW.

In a Sunday television interview Hunt said big businesses should stop warning about the economic impact of Brexit and instead get behind Theresa May.

Hunt told the BBC it was completely inappropriate for industry leaders to undermine the prime minister by speaking out about their fears.

Hunt said: "We are in an absolutely critical moment in the Brexit discussions and what that means is that we need to get behind Theresa May to deliver the best possible Brexit. What businesses want is clarity and certainty, and the more we undermine Theresa May the more likely we are to end up with a fudge, which will be an absolute disaster for everyone."

Britain is on course to end its membership of the EU next March, having first joined in 1973, but trading will continue for a limited period after that under a transitional period arrangement between the two sides.