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Let's talk about how to organise a referendum on independence or you could end up in court.

That was the message sent from the Parliament in Westminster to the Parliament in Edinburgh today.

It could herald a first and historic struggle in the Supreme Court over the respective powers of the two parliaments.

Westminster's case is simple - constitutional questions were reserved for the UK parliament. They were not devolved to Scotland.

Edinburgh's reply is that we have a right to ask the people of Scotland what we like and when we like.

David Cameron knew that his government's intervention this week would be used by Alex Salmond's government to complain of yet more lecturing from an English Tory-led government.

However, he was not prepared to allow the canniest political operator in these Isles to set the date and the question and even decide who votes for a referendum which could lead to this United Kingdom dividing.

When Alex Salmond named a date for his referendum some in Westminster claimed he'd blinked first. Scotland's first minister will claim, however, that he's not done anything he wasn't already planning to do.