Britain's Prime Minister Theresa May gives her speech on the final day of the annual Conservative Party Conference in Birmingham, Britain, October 5, 2016. [Photo/Agencies]

British Prime Minister Theresa May is seeing her plans to engineer a calm, negotiated departure from the European Union descend into uncertainty.

Analysts say the situation casts doubt over the UK's ability to strike trade deals quickly with major partners, such as China.

Officials in Beijing and London say they are ready to negotiate an agreement, but any timetable has been thrown into doubt by uncertainties.

The first of these is a challenge mounted by private individuals in the High Court, which lawyers say is one of the most important constitutional issues to come before the courts in modern times.

"You cannot underestimate the constitutional importance of this case," said one lawyer, who requested anonymity.

The private challenge is being brought by Gina Miller, a London-based businesswoman and philanthropist, and other private individuals. They maintain May does not have the right to declare Article 50－the formal notice to the EU starting the two-year withdrawal period－without first seeking approval from Parliament.

The government and supporters from the Leave campaign say May will use sovereign powers, inherited from the monarchy, to declare Article 50 without referring to Parliament. Britain does not have a written constitution, instead relying on parliamentary acts and legal precedents.

If Miller and her backers succeed, May will be forced to allow Parliament to vote on plans to invoke Article 50.

Given that 480 MPs out of the 650 backed Remain in the referendum, a successful High Court challenge could potentially throw the whole question of leaving the EU, and the manner in which it is done, into the air.

May had reluctantly agreed, ahead of a debate called by the opposition last week, that MPs would have an opportunity to debate Britain's exit, but that fell short of acknowledging their right to vote on the issue.

Lawyers say it is almost certain the losing side in the High Court will appeal to the Supreme Court, and officials there have indicated that any such appeal would be fast-tracked, with a ruling issued by the end of the year. May has said she intends to invoke Article 50 by March.