Just like his EU "veto" in December 2011 , David Cameron's promise of a referendum on UK membership has won the Tories a poll bounce. A ComRes survey  for tomorrow's Independent on Sunday and Sunday Mirror shows that Cameron's referendum pledge has boosted support for the Conservatives by five points and more than halved Labour's lead to six (although support for the latter is unchanged at 39 per cent). The rise in Tory support from 28 per cent last month to 33 per cent has come almost entirely at the expense of UKIP, which is down four points to 10 per cent. At the same time, the number of people agreeing that Cameron "is turning out to be a good Prime Minister" has risen by five points to 32 per cent, while the number disagreeing has fallen by six to 46 per cent, giving him a net approval rating of -14, his best score since June 2011.
The sudden surge in Tory support, albeit from an unusually low base of 28 per cent, will cause some discomfort in Labour circles and lead more to conclude that Ed Miliband has miscalculated by refusing to match Cameron's offer of a referendum. If the Tories are only six points behind in mid-term, who's to say they won't win the next election?
There are, however, at least two reasons why Labour shouldn't panic. First, just like the Tory poll bounce following the PM's EU "veto", the surge in support may prove to be only temporary. After a week of favourable coverage  from the media (almost all of the fieldwork took place before the negative GDP figure was released), it would be unusual if the Tories' standing hadn't improved. One of Miliband's strengths is that he isn't swayed by short-term fluctuations in the polls and I expect this occasion will prove no exception.
Second, it was always likely that a large number of UKIP supporters would return to the Conservative fold before the next general election. Cameron's promise of a referendum may merely have accelerated the process. The biggest problem for the Tories remains that they are in retreat in those areas - the north, Scotland, Wales - that denied them a majority at the last election.
Finally, it's worth remembering that just six per cent  of voters regard the EU as one of the most "important issues" facing Britain. The outcome of the next election will be determined by growth, jobs and public services - the issues that matter to most people. The promise of an EU referendum might have won the Tories back some support from UKIP but, on its own, it won't be enough to transform the party's fortunes.
Update: Part of the shift in support for the parties is attributable to a change in methodology by ComRes. At UK Polling Report , Anthony Wells calculates that without this the numbers would have been: Labour 37 (-2), Conservatives 32 (+4), Liberal Democrats 11 (+2), UKIP 13 (-1), so there would have been a slightly smaller increase in support for the Tories and a significantly smaller fall in support for UKIP.
YouGov's poll  for the Sunday Times also shows an increase in support for the Tories, who are up two points to 35 per cent, their best rating in a YouGov survey this year. Labour are down two points to 41 per cent, with the Lib Dems up two to 12 per cent and UKIP down two to seven per cent.
A Survation poll  for the Mail on Sunday has the Conservatives up two to 31 per cent, Labour unchanged on 38 per cent, the Lib Dems down one to 10 per cent and UKIP down two to 14 per cent.