A new global carbon tax and imposing a charge on currency speculators could help raise $400 billion to make up a shortfall in foreign aid spending, a United Nations study has found.

The World Economic and Social Survey report, released overnight, says rich Western countries last year cut back on the overall pot of foreign aid funds for poorer nations for the first time in years. More than $160 billion of promised assistance has not been received. Australia has slashed $3 billion from its projected spending on aid. But while the birthing pangs for Australia's carbon tax have proved difficult enough, the UN is proposing to go even further with an international scheme to make up gaps in aid spending.

It proposed charging $25 per tonne for carbon dioxide emissions in developed countries and earmarking the estimated $250 billion raised for ''international co-operation''. Australia's carbon tax rate is $23 per tonne of emissions.