The rapid expansion of Britain's international aid programme has left it increasingly exposed to corruption and fraud, the new independent watchdog has warned.

The Independent Commission for Aid Impact (ICAI) described the Department for International Development's approach to tackling fraud corruption as "fragmented" and in need of "significant improvements".

Publishing its first reports, the ICAI rated DfID's anti-corruption effort as "amber-red" on a traffic light rating system - meaning that it is "not performing well".

The commission's finding will provide fresh ammunition for critics who say the Government should not be increasing international aid at a time of painful spending cuts at home.

The ICAI said its rating was "strongly influenced" by the Government's decision to rapidly increase the aid budget to 0.7% of gross national income while channelling a greater proportion to "fragile and conflict-affected states".

"This inevitably will expose the UK aid budget to higher levels of corruption risk," it said. "Our assessment is that DfID's current organisation of responsibilities for fraud and corruption is fragmented and that this inhibits a coherent and strategic response to this critical issue.

"DfID needs to give significantly greater attention to the fight against corruption to manage this increasing risk."

The ICAI said the lack of any attempt by DfID to quantify the losses due to corruption made it difficult to assess how effective its risk management had been.

Two ICAI reports looking at specific aid projects in Bangladesh and Zimbabwe rated both programmes "green amber", meaning they were performing well but in need of some improvements.

International Development Secretary Andrew Mitchell said: "I have already changed how the UK delivers aid to ensure it is focused on tangible results on the ground but we will use these reports to identify further reforms."