Donald Tusk has bid an emotional farewell to the United Kingdom after Theresa May triggered Article 50 and begun the formal two year Brexit process.

Unless Britain and all 27 remaining European Union member states agree to extend the deadline, Britain will leave the European Union on March 29, 2019.

The future status of around three million European Union citizens living in Britain and more than one million British nationals in other parts of the bloc has been a major source of discord since the referendum.

Germany said on Wednesday it expected hard negotiations over Britain's exit from the European Union and warned London that the timeframe for the talks was "damn narrow".

A qualified majority shall be defined in accordance with Article 238 (3)(b) of the Treaty on the Functioning of the European Union.

The EU has said the two processes should come one after the other - first a deal on Brexit and then one on future ties.

He said Britain had decided that it would leave the bloc - but "the issue of how we will leave, and the democratic checks and balances along the process of the negotiations, remains unresolved". If Britain crashed out of the European Union without a trade deal it would fall back onto World Trade Organization rules, meaning tariffs and other barriers to trade.

May will now enter formal exit negotiations with European Union leaders over what the UK's future relationship with the bloc will look like.

May has said that she can only guarantee the status of European Union nationals when she receives reciprocal assurances on guarantees for British citizens.

The UK government has also said it will reluctantly have to consider reintroducing direct rule in Northern Ireland, if talks there over restoring a power-sharing executive fail.

"We're going to take this opportunity to build a stronger and fairer Britain that our children and grandchildren are proud to call home", she noted.

Britain insists that it must regain the right to control immigration and end free movement from the bloc.

May has suggested that if talks stall she could walk away, saying that "no deal for Britain is better than a bad deal for Britain". That is what worries some on the other side, because many British businesses rely on European workers.

'We know that in the treaties it is the European Parliament which has to approve the final agreement'.

Even if some terms of divorce are not agreed, the United Kingdom will fall out of the union on March 29, 2019.

May insists that Britain is not turning its back on Europe. I know the United Kingdom wants this to happen more or less at the same time. May would find it hard to meet her goals and must be accountable if she fails.

Others anxious Britain was taking a leap in the dark.

"It is in the interests of both parties to get this done as quickly as possible and as amicably as possible, " he said.