Politicians have been urged to back a "Robin Hood tax" on banks' financial transactions to raise billions of pounds to fight poverty, protect public services and fight climate change.

A coalition of charities, aid agencies, green groups and unions called on all political parties to support a global tax on banks to help tackle the "human damage" caused by the global economic crisis.

To back up the campaign, a huge message was projected onto the Bank of England building in the City of London, saying: "Be Part Of The World's Greatest Bank Job".

Money raised from a tax would help avoid cuts in public services, meet the Government's target of tackling child poverty, protect schools and hospitals and be spent on encouraging more people off benefits into work, it was argued.

In a letter to the leaders of the UK's political parties, the campaign groups said: "You could ignore the big problems facing the world, and accept that climate change will stay unchecked, and that the poorest people at home and abroad will have a very hard time of it over the next decade. Or you can find all the money needed by directly taxing the British public themselves.

"Or you can work to find an innovative, modern, regular way of accumulating a fund of money to deal with big issues boldly. 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."

Barbara Stocking, Oxfam's Chief Executive, said: "We have a once in a generation opportunity to make global finance work in the interests of ordinary people at home and abroad.

"A tiny tax on banks would make a massive difference to the millions of ordinary people around the globe forced into extreme poverty by the economic crisis."

TUC general secretary Brendan Barber said: "The crash was made in the finance sector - finance should now make a proper contribution to putting right the damage the crash caused and preventing huge cuts in vital public services."