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Tax evasion is rarely out of the headlines - especially with so many indebted governments keen to hunt down every tax dollar that is due.

At least $21tn (£13.5tn) of untaxed private wealth was invested in global tax havens in 2010, according to , a group of tax professionals which campaigns on the issue.

To put that into context - it is more than a quarter of the total GDP (gross domestic product) for the whole world - about the size of the US and Japanese economies combined.

But how credible is that figure?

James Henry, the author of the report, says: "We think these numbers are conservative and about 90% of offshore financial wealth is actually evaded tax.

"The amount that some developed countries would receive if all taxes were collected, would balance the foreign aid that those governments spend," he says.

To arrive at their figure, the Tax Justice Network looked at the offshore assets under management deposits and custody assets of the top 50 individual banks globally.

"We also built models of 139 developing countries going back 30 years - the most comprehensive models ever developed on unrecorded capital outflows and the accumulated value of all this offshore wealth," he explains.

False invoicing

Ronnie Ludwig, a partner in the accountancy firm Saffery Champness in Edinburgh and a former tax inspector with expertise in tax havens, says he could argue with the numbers in the report, but not with the sentiments it expresses.

He says the report makes a lot of assumptions, which necessitate a lot of guesswork.

"There is a real problem about lack of fiscal transparency. Governments around the world need to look at the role they play," he says.

"In some cases they conveniently turn a blind eye about what is happening on their doorsteps and in some cases actually facilitate it," he maintains.

He believes it is a problem which can only ever be resolved with a coordinated international approach.

Meanwhile, Jagdish Bhagwati, professor of Economics and Law at Columbia University in New York, says the numbers are "very fragile" and do not include the mismatching of under and over-invoicing - known as transfer pricing.

"This is when you get two sets of invoices - one from the country which is exporting and one to be shown to the country which is importing something," he explains.

The difference is money which is not reported - money which could, and should be taxed.

Labyrinth trail

"Companies will have offshore trusts, the beneficiaries of that trust may be a series of offshore companies in different locations around the world," says Mr Ludwig.

He says that directors of these companies will be based in different countries which have different jurisdictions and there will be clauses in the trust deeds which enable them to move to other jurisdictions should any awkward questions start to be asked.

"That is why it is very difficult to track these people down," he says, "let alone to collect tax."

He believes what is needed, is a comprehensive audit trail, although he says it would be a very difficult task to achieve.

But just because money is in a tax haven, it does not mean that it is illegally untaxed but, as Dr Bhagwati points out, it is difficult to differentiate between tax evasion and tax avoidance.

Not just rich people, but also middle class people, seek ways of not paying tax because they are not confident their government would spend it wisely.

"Many people in India for example, think their government is inefficient and corrupt," he says.

Pointing the finger

Mr Henry says however that what has been driving tax avoidance since the 1970s is the rise of the global private banking industry.

He speaks about the banks, which have been embroiled in scandal and which needed bailing out by their governments during the financial crisis, helping the wealthiest people on the planet set up these sophisticated offshore structures.

"To combat the problem, we need automatic information exchange," he says.

"The US does that with Canada and the UK and about 20 other developed countries, but when Mexico wanted the same information that the US was providing to Canada about offshore deposits of Mexicans, they heard nothing," he says.

"Developed countries do not share information with developing countries."

He maintains that although tax havens such as Vanuatu and the Cayman Islands were targeted, the ultimate destination for money is not the conduit islands but the large financial centres of the world.

Mr Ludwig notes that in 2008, Transparency International, which measures perceived corruption, ranked the US as having the world's most secret jurisdiction - ahead of Switzerland, Luxembourg and the Cayman Islands.

Meanwhile, Mr Ludwig reiterates that concerted effort by the international community is needed to address the problem.

"If you want to have the cure, you need to swallow the medicine," he says.

This discussion was broadcast on In the Balance, which can be heard on ����ý World Service at 11:00 GMT on Saturdays.