Labour has "missed a golden opportunity to show global leadership" with its plans for a tax on banks, campaigners have said.

Gordon Brown drew loud applause while launching his party's manifesto when he said banks would "pay their fair share to society" through an international levy.

The Prime Minister insisted working alongside the United States and European partners was the "sensible" course of action and said he was "confident" global agreement could be reached.

And he warned: "I believe if banks were consistently having to pay a tax in this country that was not being charged in other countries, it would create a problem for us in the City of London."

But campaigners in favour of a "Robin Hood Tax" said the party should have pledged to go ahead with a unilateral transaction tax in the UK.

A spokesman for the Robin Hood Tax campaign - which brings together charities, trade unions and other groups and has attracted celebrity support - said: "We welcome the inclusion of a bank tax in the Labour Party manifesto but the proposal is lacking in both ambition and detail needed for a Robin Hood Tax that is truly fair for all.

"Done properly, a Robin Hood Tax on banks' financial transactions could raise tens of billions of pounds for good causes - but it is unclear exactly how much Labour's levy would raise or whether the money would be used to tackle climate change and help poor people at home and abroad.

"The party has missed a golden opportunity to show global leadership. Robin Hood and his merry men will not be setting down their bow and arrows just yet."

Mr Brown said: "We don't believe that you can get a long-term solution unless you get a solution that involves lots of countries. We don't want a race to the bottom where different tax authorities compete with each other."

Both the Tories and Liberal Democrats have pledged to introduce a tax on banks unilaterally if international agreement cannot be found.