Negotiations on future trade relations between the UK and EU could begin as early as this autumn, European Council president Donald Tusk has indicated.

But the move, much desired by Downing Street, will only take place once "sufficient progress" has been achieved in talks on the arrangements for Britain's withdrawal.

Mr Tusk did not spell out what progress would be regarded as sufficient, but his words are likely to be interpreted as referring to the settlement of a "divorce bill" of an estimated £50 billion set to be presented to the UK.

Unveiling his draft negotiating guidelines for forthcoming withdrawal talks under Article 50 of the EU treaties, Mr Tusk insisted that Brussels will take a "constructive" approach and wants to keep the UK as a "close partner" on trade and security in the future.

Withdrawal negotiations were essentially a matter of "damage control" following a Brexit decision which had created "disruption and uncertainty", he said, predicting that talks would be "difficult, complex and sometimes even confrontational".

But he said: "The EU27 does not and will not pursue a punitive approach. Brexit in itself is already punitive enough.

"After more than 40 years of being united, we owe it to each other to do everything we can to make this divorce as smooth as possible."

Speaking in Malta, Mr Tusk said he will visit London for talks with Theresa May ahead of a special summit of the 27 remaining EU states in Brussels on April 29, when they are expected to approve the negotiating guidelines, clearing the way for talks to begin in earnest.

He issued a firm rejection of Britain's call for simultaneous divorce and trade talks.

"Starting parallel talks on both issues at the same time, as suggested by some in the UK, will not happen," he said.

But he held out the hope that talks could move on to future trade relations within a matter of months.

"It must be clear that EU, as 27, decides if sufficient progress has been achieved, probably in the autumn, at least I hope so."

While the guidelines insist that a trade deal can only be formally concluded once the UK has ceased its membership, they add that it will be possible to negotiate "an overall understanding on the framework for the future relationship" prior to Brexit to allow for a swift agreement after withdrawal.