The Opposition leader said a Conservative government would impose tougher controls to ensure development money was not misspent and show taxpayers where every penny was going.

But the temptation to switch the money to meeting domestic needs would be a "serious long-term mistake" as it would help foster terrorism, climate change and war, posing security risks at home.

The Conservatives are committed to meeting a United Nations target for countries to increase aid spending to 0.7 per cent of GDP by 2013, making it one of two areas, with the NHS, protected from cuts.

According to the party's new strategy, published yesterday, more aid money would be made dependent on results and more would be put in the hands of communities and individuals.

Spending would be overseen by an independent watchdog.

Aid charities gave a cautious welcome to the proposals but warned a plan to give aid "vouchers" to individuals could undermine health and education.

And International Development Secretary Douglas Alexander said the increased aid pledge was undermined by a failure to explicitly limit the amount used to tackle climate change.

Launching the proposals at the London headquarters of Save the Children, Mr Cameron told activists Britain was "not a country of fair-weather philanthropists".

"I understand why people might say that at a time like this, in economic circumstances like these, that increasing the amount we spend on foreign aid is the last thing we should do," he said.

"But economic difficulties at home should be the time for us to re-affirm our moral responsibilities, not reduce them. So yes – people at home are hurting in this recession. But they understand that there are still billions who have the tiniest fraction of what we have, even in a recession, and that it's our social responsibility to help them."

Phil Bloomer, Oxfam campaigns and policy director, said: "Oxfam welcomes David Cameron's leadership and clear commitment to ensuring Britain lives up to its responsibility to tackle global poverty and fulfil its aid promises.

"It is a sign of progress that all three major parties are now convinced that helping the world's poor is a moral duty they cannot ignore. Britain's help is needed now more than ever as poor people's suffer the devastating effects of the economic crisis, hunger and climate change."