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Sir Isaac Newton had it right. He may have been writing in 1687 about mechanics but his third law tells you all you need to know about the split in the Coalition on Europe.

"To every action there is always an equal and opposite reaction."

Nick Clegg's switch from defending David Cameron's negotiating stance to condemning it in the space of just two days would appear to have been a reaction to the crowing of Tory Eurosceptics and the Tory press about the use of the veto and the anger that it produced in his own party.

Senior Lib Dems, led by Paddy Ashdown and followed by Vince Cable, made absolutely clear to their leader that they did not share his understanding of the prime minister's negotiation.

Word then emerged from Brussels that the President of the European Council, Herman van Rompuy, had offered David Cameron a deal in which, in return for lowering his demands, the EU could take steps towards fiscal union without amending its treaties and without the Eurozone going it alone.

Clegg, who'd been pretty upset from the moment he heard about the veto on Friday morning, felt he had to react.

Why the delay? One of his friends puts it colourfully and developing a metaphor first used by a French diplomat*: "If your wife's been out all night you're livid in the morning but first you have to check whether she has had an accident. Once you discover she's been sleeping with your friend you're entitled to let her have a piece of your mind."

* A French diplomat compared David Cameron's negotiating position in Brussels with a man who goes to a wife-swapping party without bringing his wife.