MPs will have to vote on Theresa May’s Brexit deal without knowing the outcome of a related constitutional clash between Westminster and Holyrood.

The UK Supreme Court has announced it will not rule on an alleged Brexit ‘power-grab’ until two days after the meaningful vote in the Commons on December 11.

The Court has been considering the legality of emergency legislation passed by MSPs against the advice of the Scottish Parliament’s presiding officer since July.

READ MORE: Brexit: SNP brand Theresa May a 'liar' over Irish backstop 

It said on Thursday that its judgment would be handed down on December 13.

The announcement coincided with Europe’s highest court announcing it would turn around a decision on whether MPs had the power to halt Brexit in a fortnight.

After an oral hearing on November 27, the European Court of Justice said it would give a final opinion on December 10, on the eve of the meaningful vote.

HeraldScotland: The Supreme Court in London

The court’s top adviser has given a preliminary opinion saying MPsS should be able to unilaterally revoke the Article 50 withdrawal process, and the ECJ is likely to say the same.

However MPs will have to vote in ignorance of the Scottish dimensions.

In March, Holyrood voted 92-32 for an SNP-led Continuity Bill in a bid to block a perceived ‘power grab’ on devolved powers in the main Brexit law then going through Westminster.

The UK Government and Scottish Government were divided on how powers in devolved areas should be distributed once repatriated from Brussels after Brexit.

READ MORE: "We have a roadmap out of the Brexit shambles" - says cross-party group post ECJ ruling 

SNP ministers wanted all of the 100-plus devolved competencies to lie at Holyrood.

But the UK government wanted 24 to stay for several years at Westminster, including some covering agriculture, fishing and procurement to protect the UK single market.

The Continuity Bill was intended as a backstop, to be used if the two sides failed to agree the distribution of powers, which would transfer devolved EU law into Scots law at Brexit.

The Bill was the first of its kind under devolution, and the first passed by MSPs in defiance of an advisory ruling by Presiding Officer Ken Macintosh that it was legally incompetent.

Mr Macintosh said the Bill exceeded Holyrood’s powers as it strayed into EU law, however the Government’s top law officer, the Lord Advocate James Wolffe QC, said it did not.

After Holyrood passed the Continuity Bill, the UK Government swiftly announced it would challenge it at the UK Supreme Court and attempt to have parts of it struck down.

The two governments argued their positions at the Court over two days in late July, with Mr Wolffe pitted against the Advocate General for Scotland, Lord Keen.

However since then almost nothing has been heard of the matter.

The Scottish Government declined to comment on the timing of the court's decision.

A spokesperson said: "The Continuity Bill was enacted to prepare Scotland's law for Brexit in the manner considered appropriate by the Scottish Parliament. 

"We await the Supreme Court's judgment."