The European Union's chief negotiator has warned the UK it will have just 18 months to agree a Brexit deal with the organisation.

Michel Barnier told reporters in Brussels "time will be short" to reach an agreement after the Prime Minister triggers Article 50.

Theresa May has promised to begin the exiting process by the end of March.

The senior EU official said he expects a deal to be reached by October 2018, just 18 months after the Prime Minister begins the legal process.

Barnier said: "Time will be short. It is clear the period for actual negotiations will be shorter than two years.

"At the beginning, the two years included the time for the council to set guidelines and to authorise negotiations.

"At the end, the agreement must of course be approved by the council and European Parliament.

"Finally the UK will have to approve the agreement - all within the two-year period."

Barnier added: "All in all there will be less than 18 months to negotiate. That is short. Should the UK notify by the end of March as Prime Minister Theresa May said she would, it is safe to say negotiations could start a few weeks later and an article 50 reached by October 2018."

The chief negotiator also warned the UK that exiting nations cannot cherry pick what aspects of the single market it wants to retain.

He said: "Being a member of EU comes with rights and benefits. Third countries (those not in the EU such as the UK) can never have the same rights and benefits since they are not subject to the same obligations.

"The single market and its four freedoms (which include freedom of movement) are indivisible. Cherry picking is not an option."

Under the Treaty of Lisbon, the UK and the EU will have two full years to get an exit deal agreed before Britain formally leaves. The negotiation period can only be extended if all other EU member states agree to it.

The UK Government cannot begin its negotiations to leave the organisation until Article 50 has been triggered.

The Supreme Court is currently hearing an appeal on behalf of Downing Street over the Prime Minister's authority to begin the process.

Number 10 argues May should be able to trigger Article 50 without needing prior consent from MPs.

Last month, the High Court ruled against the government.

A verdict is expected to be reached by the Supreme Court in January after a four day hearing this week.