NEW YORK (Fortune) -- Finding a tax haven is easy. Just flip to the back of the Economist. "New Accounts in 8 Minutes" brags one ad. Another promises that no one is "better positioned to deliver solutions that work" on offshore companies. And yet another offers more than 20 years of experience and the "best prices guaranteed."

Those boasts might not be possible much longer. The G-20 meeting in London Thursday is expected to produce an agreement on how to compel tax havens to sign international codes of conduct, potentially ending years of secrecy, minimal regulation and the stash of as much as $11.5 trillion, according to the Paris-based Organization for Economic Cooperation and Development.

"There is broad agreement among the G-20 that it's important to bring the offshore financial centers into the overall community of the global economy and under appropriate rules of the road," said Mike Froman, an Obama advisor for international economic affairs in a conference call with reporters March 28.

Tax havens have drawn the scorn of large countries for years, but little has changed. Their minimal or nonexistent taxes deprive governments of tax revenue from individuals and businesses - estimates are as high as $255 billion annually for individuals worldwide, according to the Tax Justice Network - and their secrecy laws can shield financial crime.

In the U.S., an estimated $40 billion to $123 billion in taxes is lost each year, according to a recent U.S. Treasury report. The GAO found that 83 of the 100 largest publicly traded U.S. corporations have subsidiaries in tax havens.

But tax havens are under the gun now because of the global economic downturn and multi-billion dollar financial scandals. In February, Swiss banking giant UBS admitted to helping customers evade taxes, paying the U.S. $780 million in fines and revealing hundreds of American clients.

Antigua-based Stanford International Bank is under investigation after U.S. authorities accused financier Allen Stanford of leading an $8 billion financial fraud. And disgraced money manager Bernard Madoff, who pleaded guilty to what authorities say was a $64.8 billion Ponzi scheme, may have used Gibraltar, Isle of Man and other tax havens to stash funds, according to a court-appointed trustee for his assets.

The U.S. effort

Bowing to pressure, traditional offshore destinations like Switzerland, Belgium, Andorra and Liechtenstein have promised to increase financial transparency. Offshore centers have recently signed more than 25 tax information exchange agreements with other countries, the OECD says.

But those reforms could take years to implement, so the U.S. is drafting its own regulations to speed things up.

In March, Senator Carl Levin, D-Mich., introduced "The Stop Tax Haven Abuse Act," which would target a list of "secrecy jurisdictions" and make U.S. citizens more responsible for proving that their offshore tax arrangements are legitimate. It would also increase reporting requirements, step up penalties and enforcement and close offshore trust and tax dividend "loopholes."

"Offshore tax havens have declared economic war on honest American taxpayers by helping tax cheats hide income and assets and thereby depriving the U.S. Treasury of $100 billion annually in tax revenues," Levin said in a statement to Fortune.

Levin's bill and similar legislation from Rep. Lloyd Doggett, D-Texas, are backed by the White House, the Treasury, and the Internal Revenue Service. The White House has also said it will raise $210 billion over ten years from stronger international tax enforcement.

"It's excellent," Lucy Komisar, a founder of advocacy group Tax Justice Network-USA, said about Levin's effort. "He's putting the onus on the other side," she said. "We know that there really isn't any other reason to put companies and money offshore."

But some disagree with Levin's approach. "Increasing resources for enforcement and increasing the emphasis on information sharing provisions is a great strategy," said Mihir Desai, professor of finance at Harvard Business School. "But blacklisting countries can deteriorate into a game of whack-a-mole and identifying and punishing countries selectively on dubious criteria seems ill-founded."

Not a list they want to be on

Some countries resent being branded as tax havens. Bermuda is listed in the Levin legislation as one of 34 target "offshore secrecy jurisdictions," but the nation denies it's a tax haven, citing tax information sharing agreements with the U.S., U.K. and others, as well as a history of assisting with U.S. tax-evasion prosecutions.

"People are looking to attribute blame for a terrible situation that's happened in the financial markets," says Greg Wojciechowski, President and CEO of the Bermuda Stock Exchange. "But when you're looking down through what's caused it, you also have to recognize when there isn't a weak link in that chain. I think it will come out that Bermuda is not that weak link."

And some wonder if the Levin bill will just make for more paperwork. Roger Lorence, a partner specializing in international taxation at law firm Sadis & Goldberg, says only the honest will comply with the new reporting requirements. "The dishonest person does not fill out this long form and say 'Oh, hi, I'm committing fraud through my secret accounts.' It's silly."

Despite such criticism, G-20 members are ready for action. "More progress is needed," says Grace Perez-Navarro, Deputy Director of the OECD's Centre for Tax Policy and Administration. "The G-20 need to give a strong message to all financial centers that a rapid implementation of the standards is now required."