David Cameron's commitment to the fight against global poverty is called into serious question today as a group of Britain's leading aid experts say his party seems more interested in "political positioning" than helping the world's poor.

In a letter to the Observer, the group, which includes the former chairmen of Make Poverty History and Oxfam GB, also says the Tories seem to be pushing "crude attempts to export failed ideological or populist policies" on aid.

Their intervention will embarrass the Tories as the leaders of the main parties attend events to mark International Development Day.

The Tories have committed themselves in their election manifesto to the UN target of spending 0.7% of national income on international aid by 2013. But they have angered aid agencies with plans to set up a new stabilisation and reconstruction force within the armed forces that would carry out aid work and infrastructure projects in the aftermath of combat.

The aid experts voice fears that aid money will be diverted to the military and away from the fight against poverty. "The welcome shift in Conservative policy to back the 0.7% promise in 2005 has been much vaunted by David Cameron – but despite repeated requests they have refused to clearly commit to ensure aid is not diverted for other purposes," they write.

People working for charities are barred from making party-political statements during election campaigns – but the intervention of eminent former leaders in the sector reflects the views of many now working in the field.