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How much does the UK spend on foreign aid? Many think the answer is about 15% of gross national income (GNI). It's a myth many seem happy to peddle. The correct answer is 0.54%. That is £5.40 for every £1000. Absurdly generous in a world where four billion people scrape a bare existence on less than $2 a day? I don't think so.

Since 1970 the suggested figure for foreign aid has been 0.7% of GNI. During the Blair and Brown years, Britain's aid programme started to get a lot more generous and the manifesto of all the major parties in 2010 included the commitment to reach that target by 2013 and enshrine it in law. David Cameron's enthusiasm for helping the poorest at home and abroad formed part of the campaign to detoxify the Tory brand. In government, despite a largely aid-sceptic press and much grumbling from the Tory right, he has stuck to his guns. "I don't think 0.7% is too high a price to pay for saving lives," he insisted last July.

Yet in the Queen's Speech yesterday there were warm words about the target but no legislation to embed it. The big aid organisations, pleased at the ringfencing of the budget and unwilling to rock the boat, expressed muted disappointment. But this is a real blow, not only because it breaks a promise made in the Coalition agreement, but because it would have acted as a bulwark against future cuts. Now there is nothing to stop the next administration doing exactly what Australia did yesterday in cutting around £2 billion in foreign aid over the next three years. This amounts to balancing its books at the expense of the world's poor.

The importance of UK legislation would be in the strong signal it sent out. Britain is already top of the G8 class on international development, comfortably ahead of France and Germany and way beyond backsliders Italy. Only the smaller Scandinavian economies do better. The danger is that if Britain cuts foreign aid, other countries will do the same.

This is a crucial test of Mr Cameron's leadership just as he lands the chairmanship of the UN committee that will set new targets to follow the eight millennium goals, due to expire in 2015. Several of them, including the pledge to halve extreme poverty, have been missed by miles and the 0.7% pledge is seen as key to restoring credibility in the whole idea.

The British public remains to be convinced. A survey by the think-tank Chatham House found 57% favoured cutting foreign aid. (The favourite target for extra spending was the armed forces.) The Government needs to provide taxpayers with more evidence of how well-targeted development spending not only saves millions of lives but lifts families from poverty and makes them more resilient.

Partly thanks to Clare Short, the first International Development Secretary, British aid has become more adept at bypassing venal governments and concentrating on programmes that work. The Department for International Development is currently investing around £500m a year in tackling malaria, which still kills a child every 60 seconds. Yet with cheap and simple measures like treated bednets, antimalarial medicines and insect sprays, it can be beaten. In Zambia malaria deaths in children under five have fallen by nearly two-thirds. If budgets are cut, more children will die. Currently spending 2p a day, per British person is saving a life every three minutes.

Success stories, such as big rises in girls going to school and drops in women dying in childbirth, need to be trumpeted. So does value for money. For the cost of training a single guide dog, enough Trachoma sufferers could be cured to prevent 2600 years of blindness, according to the charity Giving What We Can.

This isn't to say that some aid doesn't work. It's just that much, if not most, does work, as Robert Cassen argues in Does Aid Work?, the most exhaustive study of foreign aid ever undertaken. Putting a figure on a foreign aid budget is easier than putting a figure on its results. That is because the results are priceless.

Anne Johnstone is a member of the Oxfam Scotland Advisory Board.