Expatriate Americans are renouncing U.S. citizenship in record volumes because of increasingly onerous tax-filing requirements—and the number doing so is likely to continue rising.

Around 6.32 million Americans (excluding military) live abroad—about as many as who inhabit the state of Tennessee. The majority of expats are based in Europe, but growing numbers live in the Asian financial hubs of Tokyo, Singapore and Hong Kong.

These émigrés have been swept up in a government crackdown on tax evasion, following well-publicized investigations into Swiss banks like UBS and Credit Suisse that are alleged to have helped Americans conceal money abroad.

The result is that increasing numbers of expats are opting to relinquish their passports.

In the first six months of 2014, 1,577 Americans expatriated their citizenship, according to data from the IRS— the U.S. tax authority.

Last year as whole, 2,999 Americans officially cut their ties to the U.S.—over three times more than in 2012 and far above a previous record of 1,781 in 2011.

“We probably get two or three calls a day from people saying they want to relinquish citizenship. That would have much rarer six or seven years ago, maybe once every couple of months,” Scott Michel, the president of tax law specialist Caplin & Drysdale, told CNBC. Michel was one of several lawyers and financial advisers across the U.S., Europe and Asia who told CNBC that more Americans were requesting advice on expatriating.

“What is really driving Americans to expatriate is not that they do not want to pay taxes. What is driving them crazy is that now filling out tax returns is much more complicated—it requires tremendous work in terms of tax-record keeping and then it can cost thousands of dollars to get a competent person to fill it (the tax return) out,” David Kuenzi, the founder of Thun Financial Advisors, told CNBC.

A particular bugbear is the Foreign Account Tax Compliance Act (FATCA), which came into force at the end of June this year with the intention of combating tax evasion. It requires all Americans to provide the IRS with detailed information on their foreign financial accounts and offshore assets, and also requires that foreign financial institutions divulge any accounts held by Americans, as well as any entities in which Americans have financial interest.

This latter point has proved controversial, with some countries claiming it violates local laws, and expats saying that certain banks are now unwilling to service them due to the increased administrative burden and the risk of misfiling to the IRS and being hit by penalties.

Individuals can also be hit by steep penalties for misfiling, even by accident. Fines start at $10,000 and criminal charges can apply if the IRS decides information was withheld “willfully”.

“My filing requirements and possible penalties are very much different (to U.S. residents),” Jim Rogers, a renowned U.S. investor who lives in Singapore, told CNBC. “It is treating Americans who live abroad differently—you would think it is some kind of discrimination.”

Rogers, who was born in Baltimore, said that relinquishing citizenship “crosses the minds of everybody lives abroad”.

“I know three or four Americans who are basically being forced to give up citizenship. If you make simple mistakes it could cost you—it could be a criminal offense, or at least a very financially onerous one,” he told CNBC.

The U.S. is the only country in the world which levies taxes on the basis of citizenship rather than residency—meaning people can be liable to the IRS without ever having lived in the country, if they hold a U.S. passport or green card.

CNBC spoke to one such citizen—who asked to remain anonymous due to the “very long arm” of the IRS—who was born and raised in London but holds a U.S. passport through his American father.

A trainee obstetrician, he mulled cutting his U.S. ties after his sister was threatened with a hefty fine for accidentally misfiling.

“Giving up citizenship would be incredibly difficult to undertake from an emotional standpoint,” he told CNBC. “However, financially you have to be ever so careful…It is something I have to consider in the future.”

Renouncing U.S. citizenship is neither easy nor cheap however. Citizens must undergo at least two intensive interviews with consular officers and renunciation is subject to final approval by lawmakers in Washington.

Would-be expatriates must also pay both a fee and an “exit tax”, although the latter only applies to those who meet a minimum income or net worth threshold, or have failed to comply with federal tax obligations.

This month, the fee for renunciation will rise five-fold to $2,350. Justifying the increase, the Department of State noted that demand for expatriation had increased “dramatically” since a fee of $450 was set in 2010.

U.K. expats may also soon feel the long arm of the taxman. Last month, the British government launched a consultation on whether non-residents should cease to benefit from the £10,000 ($16,100) tax-free allowance which all Britons are currently guaranteed. The move was criticized by the U.K. media, with the Daily Telegraph newspaper headlining with “Expats face £400 million tax raid”.

By CNBC’s Katy Barnato