As Google and Apple are probed on tax avoidance, it's time for political leaders around the world to take a stand and stamp the practice out

Sometimes charities have to do a bit of getting on their soap box and moralising to be noticed, but luckily for me, on this one the numbers speak for themselves.

With the help of our resident tax geek - Matti Kohonen - Oxfam has crunched the numbers from the Bank of International Settlements, Credit Suisse’s Global Wealth Databook and other sources to calculate how much wealth is held offshore by individuals and how much tax we are all losing as a result. The figures are mindboggling.

We have found that people are stashing at least £12 trillion offshore. And as a result, the tax revenue we are losing globally is more than £100 billion. As well as being a lot of zeros, that's enough to end extreme poverty twice over.

"That old chestnut" I hear some of you cry (we’ve claimed a few things could end poverty recently), but the fact is that it is a compelling illustration of what such a colossal sum could do. With one in eight people around the world going hungry, women dying every day in childbirth in the poorest countries, and hundreds of thousands of people right here in the UK having to resort to food banks there’s certainly no shortage of need. Whatever you care about, whatever you’re worried about being starved of funding, I bet £100 bn could pay for quite a lot of it.

As well as calculating how much of people’s money is sloshing around idly in tax havens, we've also been able to look at who can change this. And those numbers tell a pretty strong story too. One third of the lost tax money is being facilitated by UK-linked tax havens - the Overseas Territories and Crown Dependencies. And whilst the UK has reached out to some of these territories to share tax information with us – a crucial step to making sure people and companies are taxed fairly – it doesn’t yet do anything for the poorest people.

This week David Cameron has written again to UK tax havens, and today the government has confirmed that they strongly support the ‘Multilateral Convention’ – a convention that gets signatories - who can include developing countries - to share vital tax information. Extending this convention to tax havens, and making sure that there will be consequences if they don’t play ball, is exactly what we are looking for ahead of the G8. And there are just a few weeks to go.

Taking a slightly less parochial view, with David Cameron in Brussels today at a meeting of EU leaders, of course this is not just a story about the UK. European tax havens (including ours) account for two thirds of the tax dodging problem. The right people are in the room today - it is time for them to act. As well as getting all tax havens signed up the Multilateral Convention, we need a blacklist of tax havens agreed by European nations, and we need a public registry of the ultimate owners of all assets held offshore.

Public opinion is leading the way on this one. My taxi driver this morning told me that he thinks it's unfair that the wealthy have accountants who can help them get away with paying the bare minimum of tax. So even though we're number crunching here in Oxfam, I do come back to the soap box in the end. A tax system that is rigged in favour of the privileged, whilst ordinary families struggle to get by – both here and in the poorest countries - is not a tax system people will stand for any longer.