The coalition of charities, aid agencies, green groups and unions is calling for a "new deal between banks and society", arguing that by taking an average of 0.05% from "speculative" banking transactions, hundreds of billions of pounds would be raised every year.

Backed by organisations such as Oxfam, Unicef and the TUC, the campaign launched a Web site yesterday featuring a video directed by Richard Curtis and starring Bill Nighy.



The group has also tapped Twitter, facebook and flickr for a band of "Merry (wo)men" and is asking people to sign up to support the idea on its site. Supporters can also download Robin Hood tax masks and posters.

The prospect of a "Tobin" global financial transactions tax has been gaining credibility in recent months after prime minister Gordon Brown floated the idea at a G20 meeting in Scotland in November.

Angela Merkel and Nicolas Sarkozy have both voiced support for the idea and last month the World Economic Foundation gave its backing to a levy on financial institutions to be used to help bail out banks in any future crisis.

The Robin Hood campaign is calling for half of proceeds of a tax on transactions between banks to be used by governments domestically and half abroad, with a 50-50 split between poverty reduction and fighting climate change.

In a letter to leaders of the UK's political parties, the group says: "We would ask you seriously to consider the Robin Hood Tax as that radical new option - a small tax on bankers that would make a huge difference to the UK, to the poorest countries and to our planet. Let's turn the crisis for the banks into an opportunity for Britain and the world."