LONDON (Reuters) - Britain's Supreme Court is likely to make its ruling in the New Year on the issue of whether the government requires parliamentary approval before launching the formal process of leaving the European Union.

Prime Minister Theresa May wants to trigger Article 50 of the EU's Lisbon Treaty, the formal step that will start divorce proceedings, by the end of March, but the High Court ruled last week that the decision had to be made by parliament.

The Supreme Court said it had granted the government permission to appeal against the High Court judgment and set aside four days, from Dec. 5 to 8, to deal with the matter.

It did not expect to make an immediate ruling on the issue.

"Judgment will be reserved at the conclusion of that hearing and follow at a later date, probably in the New Year," the court said in a statement.

That timetable casts further doubt on May's proposed timetable. The government has insisted since the High Court ruling last week that it was sticking to its target of triggering Article 50 by the end of March.

If the government's appeal fails, it faces the prospect of weeks and perhaps months of delays as parliament debates when and how to launch negotiations on the terms of Brexit.

The Supreme Court said all 11 of its justices would sit on the appeal panel, which would be chaired by its President Lord Neuberger.

Earlier, Scotland's First Minister Nicola Sturgeon said the Scottish government would apply to intervene in the legal case about Article 50, which could further complicate matters.

The United Kingdom as a whole voted to leave the EU in a referendum in June, but Scotland and Northern Ireland voted to remain in the bloc. Sturgeon has repeatedly said she did not want her country taken out of the EU against its will.

The government has said it was focused on the Supreme Court appeal and was confident that it had strong legal arguments to overturn the High Court judgment.

The Supreme Court said the exact duration and timings of the appeal hearing will depend on further submissions received from the parties on the precise legal arguments to be considered, the number of interveners and whether any other related cases are joined to this one.

However, it said that at this stage it expected the hearing to last all four days from Dec. 5.

(Reporting by Estelle Shirbon; editing by Guy Faulconbridge)