Tory and Liberal Democrat cabinet ministers are joining forces for a drive to keep Britain in the European Union.
Conservative "big beast" Ken Clarke and Lib Dem Chief Secretary to the Treasury Danny Alexander will spearhead a "patriotic fightback" against eurosceptics.
Labour's former business secretary Lord Mandelson is also due to speak at the launch of the Centre for British Influence campaign.
The cross-party push comes after David Cameron set out plans last week to hold an in-out referendum by 2017.
The Prime Minister said he wanted to renegotiate the UK's relationship with Europe, and then put the new package to a public vote.
The speech delighted many on the Tory right who have been demanding looser ties, but also drew warnings that Britain was "sleepwalking" towards an EU exit.
Tory strategists are planning to make the issue a key dividing line at the next general election, claiming Labour and the Lib Dems are trying to stop people having their say.
Mr Clarke, a former chancellor and home secretary, and currently minister without portfolio - is the party's leading europhile but has kept a low profile since Mr Cameron's speech.
He is expected to highlight the benefits of membership today, and challenge misconceptions among the general public.