Britain should expect to be faced with “complexities and difficulties” as it tries to extricate itself from the European Union, Theresa May has been warned, as Downing Street continued to put a positive spin on the Brexit process.

In a 20-minute meeting before the official start of an EU summit in Brussels, senior members of the European Parliament told the Prime Minister they nonetheless want to work towards a “viable solution” in the coming months and years.

The UK has promised to trigger Article 50 and formally announce its intention to leave the EU by the end of March, after which it will have two years to both agree both the “divorce deal” and come up with a new trade deal with the remaining members of the bloc.

Downing Street sources said Ms May spoke to European Parliament president Martin Schulz and the former Belgian Prime Minister Guy Verhofstadt, who has been appointed the Parliament’s chief Brexit negotiator.

Mr Schulz and Mr Verhofstadt said they wanted a "constructive process, while we recognise there may be complexities and difficulties at times, because that is the nature of any negotiation, overall we should keep our focus on: how do we get to the right outcome at the end”.

Footage from the start of the official meeting of the European Council - which consists of the heads of state of each member - showed Ms May standing on her own.

Later, the other 27 leaders will meet for a dinner to which Ms May has not been invited. Once she has left the summit, they will hold informal talks about how the bloc is to approach Brexit as a united front.

As she arrived in Brussels for the regular EU gathering, Mrs May said: "I welcome the fact that the other leaders will be meeting to discuss Brexit tonight.

“As we are going to invoke Article 50, trigger the negotiations, by the end of March next year, it's right that the other leaders prepare for those negotiations as we have been preparing.

"We will be leaving the EU, and we want that to be as smooth and an orderly process as possible. It's not just in our interests, it's in the interest of the rest of Europe as well."

She ignored repeated questions about private warnings to the Government from Britain's EU ambassador Sir Ivan Rogers that a post-Brexit trade deal could take a decade to finalise and even then may fail to get ratified by member states.

Number 10 said Sir Ivan, who spoke to Ms May earlier, was passing on the views of other EU nations.

"Ivan is there to report the views of others, he is doing the job of an ambassador," a source said.

"He was representing what others are saying to him."

Downing Street continues to suggest the Government believes it will be possible to complete both the required deals within the two-year time frame set out under Article 50 of the EU treaties.

"The intention is that we will have a deal within the time frame we have set out which sees us exit the EU and allows us to trade with and operate within the single market," said a Number 10 spokesman.