The BBC reports that deputy prime minister, Nick Clegg, who is leader of the pro-European Liberal Democrat party with whom the Conservatives have formed a coalition government, was not in the House of Commons during the prime minister’s explanation of his EU veto:

There is no sign of his pro-European Deputy PM Nick Clegg, who usually sits alongside the PM in the Commons.
Labour leader Ed Miliband questioned why Mr Clegg was not in the Commons, saying the PM could “not even persuade” his deputy of the merits of his actions.
BBC Political Editor Nick Robinson said he had been told Mr Clegg – who said on Sunday the result “was bad for Britain” – had decided that his presence in the Chamber would be a distraction.
The statement began with Labour MPs shouting “where’s Clegg” – and later during the statement Tory backbencher Nadine Dorries accused the Lib Dem leader of “cowardice”.

It is no secret that the coalition government is a marriage of convenience, nor that its constituent parties disagree fundamentally over Europe. But in all likelihood, Clegg’s absence from the chamber is designed more for internal consumption than a sign of the imminent coming apart of the government. From the outset, Nick Clegg has fought accusations that he is very much the junior partner in the marriage (which, well, he is), and has come repeatedly under fire from his own party for acquiescing with David Cameron on policies that the rank-and-file Lib Dems cannot stomach — most notably the rise in college tuition fees earlier in the year.

By staying away, Clegg is sending a message that he is unhappy with the prime minister and perhaps attempting to buy a little time, without providing the evening news with images of the two men sitting next to each other while the prime minister defends a decision with which everybody knows his deputy is uncomfortable.