The UK Independence Party leader Nigel Farage has claimed the prospect of an in/out referendum was his party's "greatest victory to date."
David Cameron's critics say his referendum pledge is less about national interest and more about dealing with his own part's Europe problems.
Deputy political editor Chris Ship rounds up the party political reaction to today's speech:
To the surprise of many Lib Dems, Nick Clegg appears to have played down the importance of the Coalition's divide on an in-out referendum.
Business leaders in Davos are worried about the Prime Minister's pledge to put Britain's membership of the European Union to a referendum.
With some in France keen to "roll out the red carpet" for a British exit, Germans favouring decentralisation welcome EU renegotiation talks.