A thousand economists have written to G20 finance ministers meeting in Washington, urging them to tax City speculators to help the world's poor.

In a show of unity rare in the economics profession, the experts from 53 countries describe the so-called "Robin Hood tax" or Tobin tax on transactions in financial markets as "an idea that has come of age".

Signatories include Jeffrey Sachs, director of the Earth Institute at Columbia University who is an influential adviser to Ban Ki-moon, the secretary general of the United Nations; Dani Rodrik from Harvard, and Ha-Joon Chang, from Cambridge.

The economists argue that even if such a tax was levied at just 0.05%, it could raise hundreds of billions of dollars, which could be ploughed into development projects.

"The financial crisis has shown us the dangers of unregulated finance, and the link between the financial sector and society has been broken. It is time to fix this link and for the financial sector to give something back to society," the letter says, adding that a Robin Hood tax is "technically feasible" and "morally right".

French president Nicolas Sarkozy, who is chairing the G20, has commissioned billionaire philanthropist Bill Gates to examine innovative ways to fund development, and France and Germany are known to be keen on the idea of a financial transaction tax.

With many countries on course to miss the aid promises made at the Gleneagles summit in 2005, a transaction tax is seen as a potential new source of income.

Max Lawson, spokesman for the Robin Hood Tax Campaign, said: "If the G20 don't want to listen to campaigners then they should listen to the experts. Economists have a reputation for not being able to agree on anything so the fact that a thousand are calling for a Robin Hood tax is remarkable."

A recent poll carried out by Oxfam found that 51% of the public in the UK supported the idea of a financial transaction tax, with 17% opposed.

The letter to the G20

Dear G20 Finance Ministers and Bill Gates,

We write to you as the call for a Financial Transaction Tax is now gathering global momentum, and the French government has made it a key priority for their G20 presidency.

This tax is an idea that has come of age. The financial crisis has shown us the dangers of unregulated finance, and the link between the financial sector and society has been broken. It is time to fix this link and for the financial sector to give something back to society.

Even at very low rates of 0.05% or less, this tax could raise hundreds of billions of dollars annually and calm excessive speculation. The UK already levies a tax on share transactions of 0.5%, or ten times this rate, without unduly impacting on the competitiveness of the City of London.

This money is urgently needed to raise revenue for global and domestic public goods such as health, education and water, and to tackle the challenge of climate change.

Given the automation of payments, this tax is technically feasible. It is morally right.

We call on you to implement the FTT as a matter of urgency.

Yours.