Business leaders today demand that whoever wins the keys to Downing Street on June 8 sets out their Brexit negotiating priorities within days of the General Election.

The Institute of Directors (IoD) warned that firms need time to prepare.

Cash-strapped companies could even require tax breaks to help them deal with the cost of planning for Brexit, the IoD said.

The call comes after the leaders of the other 27 European Union countries approved tough guidelines for their chief negotiator, Michel Barnier.

He has called for talks to begin less than a fortnight after the General Election, on June 19.

Theresa May has said that deadline means there is no time for a new government to get up to speed before negotiations begin.

But the IoD said that all political parties should start planning now.

The organisation calls for a detailed series of documents listing the preferred outcome list of Brexit talks to be published immediately after the election.

The information is needed to help businesses get ready for Brexit, it warns.

Preparing for every possible eventual outcome would be too expensive, the IoD warns.

It also calls for a joint UK/ EU customs committee to prevent “chaos”.

UK regulators should also be beefed up now so that they are ready when powers begin to return from Brussels.

And some some firms could need tax relief to deal with the cost of Brexit planning.

Allie Renison, Head of EU and trade policy at the IoD, said:“These negotiations not like any normal trade agreement. The scope for disruption is significant, so it would be better if the next government was as up-front as possible about their vision of Brexit. The EU has been clear about how it sees the process panning out, so after the election whoever is in No 10 should quickly produce their own detailed objectives. We believe there is enough common ground, and good will, on both sides for the talks to be constructive. But more emphasis needs to be placed on the desired shape of our future relationship, rather than just reiterating red lines.

“Businesses know that no politician can promise certainty at the moment, but IoD members are looking for more specifics in areas like the future customs agreement, and whether we will continue to be part of trade deals signed by the EU in future. The sooner after the election the government can spell out their priorities, the easier it will be for companies to plan.

“Planning is not cheap of course, and one practical step the next administration could take would be to create a Brexit voucher scheme or tax relief expansion to help firms potentially at risk. Allowing businesses to prepare will be key to a smooth Brexit.”

Talks have to be finished by Brexit Day, in March 2019.

However, Mrs May has already conceded that it could be impossible to agree all aspects of Brexit by then.