We urge G20 leaders meeting in Cannes later this week to back the efforts of French President Nicolas Sarkozy to introduce a tax on financial transactions to help low-income countries hit by the economic crisis and to protect poor people from climate change.

While the international community has been understandably occupied by financial turmoil in Europe and on efforts to boost economic recovery, it is important that we do not forget the needs of the world’s poorest.

The World Bank estimates that 64 million people have been pushed into extreme poverty as a result of the financial crisis triggered by the collapse of Lehman Brothers three years ago. And research for Oxfam suggests that falls in remittances, export earnings and tax revenues have left low-income countries with a $65 billion hole in their budgets. This problem is exacerbated by reduced aid flows as many rich countries struggle to balance their own budgets.

Meanwhile, the gathering pace of climate change has left millions more people increasingly at risk of hunger, homelessness and human suffering caused by changing weather.

A global “Robin Hood tax” on financial transactions could raise as much as $400 billion for good causes. But failure to reach global agreement should not be used as an excuse for inaction, instead willing countries should press ahead with their own tax. Critics should not pretend this cannot be done. A number of countries, including the United Kingdom, the United States and Brazil plus Hong Kong, already have their own financial transaction taxes.

The summit in Cannes offers the G20 the chance to show it can assume the mantle of global economic leadership during these difficult times. Backing (Bill Gates’ recommendation of) a financial transaction tax to help the world’s poorest would show our leaders are ready to rise to the challenge.