BUSINESS leaders have voiced concern about the slow pace of Brexit negotiations, warning it could affect a constructive exit from the EU.

Umbrella group BusinessEurope said companies needed certainty and time to prepare for future arrangements.

A statement said: "The slow pace of the negotiations is of special concern to business as it might jeopardise an orderly and constructive exit of the UK from the EU."

However, a UK Government spokesman said: "In recent weeks we have published a raft of papers setting out our position on withdrawal issues and our future partnership, with the view to progressing the current negotiations as swiftly as possible."

In a related development, more than 100 companies, with more than one million workers in the UK and EU, have signed a letter to Brexit negotiators David Davis and Michel Barnier, stressing the importance of making progress on a transition deal.

The letter said: "Our businesses need to make decisions now about investment and employment that will affect economic growth and jobs in the future.

"Continuing uncertainty will adversely affect communities, employees, firms and our nations in the future. Businesses across the EU and UK are clear: being able to plan for a transition of up to three years that avoids a cliff edge is critical for our collective prosperity.

"We are therefore writing to urge both sides to be pragmatic and determined to move to the next stage of the negotiations.

"Until transitional arrangements can be agreed and trade discussed the risk of 'no deal' remains real and has to be planned for, with inevitable consequences for jobs and growth on both sides."

Meanwhile, Boris Johnson has insisted he is "all behind Theresa for a glorious Brexit" after setting out a 4,000 word vision of Britain's future outside the European Union that has been viewed as a challenge to her leadership.

The Foreign Secretary revived the widely-criticised claim that quitting the EU could boost NHS coffers by £350 million a week and warned against paying for access to European markets in the future.

But the detailed assessment of life after March 2019 was released just six days before May sets out her Brexit blueprint in a speech in Florence, fuelling speculation about Johnson's leadership ambitions.

Johnson suggested continued membership of the single market and customs union would make a "complete mockery" of the referendum result.

He insisted Brexit would allow the UK to "be the greatest country on earth" and "our destiny will be in our own hands".

"This country will succeed in our new national enterprise, and will succeed mightily," he wrote in a London paper.

The Leave campaign's most eye-catching pledge during the referendum campaign was a claim that ending Britain's contributions to the EU would free up an extra £350 million a week that could be spent on the NHS.

But it was widely derided in the weeks after the result, and Johnson and other campaigners appeared to distance themselves from the promise.

However, Johnson now says: "Once we have settled our accounts, we will take back control of roughly £350 million per week. It would be a fine thing as many of us have pointed out if a lot of that money went on the NHS, provided we use that cash injection to modernise and make the most of new technology.”

However, Scottish Tory leader Ruth Davidson, a long-standing critic of Johnson, appeared to attack the Foreign Secretary for the article in the wake of the terror attack.

She tweeted: "On the day of a terror attack where Britons were maimed, just hours after the threat level is raised, our only thoughts should be on service."

Sir Craig Oliver, former director of communications at Downing Street for David Cameron, said even if the genuine intention was to support the Prime Minister, it was obvious it would be seen in Westminster as a "direct challenge".

He said: "I think he is reminding people that he is there and he is a force in the land, a force to be reckoned with."