For patients who took their bunions and blisters to Dr. Gordon Barrington John, the Brooklyn podiatrist’s Atlantis Footcare offices were centers for foot relief. They were also havens to hide money.

John, 45, pleaded guilty yesterday to laundering more than $200,000 in income over a three-year period, a class d-felony charge that could bring with it a seven-year prison term.

Sentencing is scheduled for September 13.

“This is one foot doctor that got the boot,” District Attorney Robert Morgenthau quipped at a press conference.

Messages left for John at his 319 Utica Ave. podiatry office were not returned. Upon learning that her boss had pleaded guilty to money-laundering, a secretary said the tax-dodging doctor had been missing all day.

“I knew something was up,” she said. “I better get…out of here!”

John’s guilty plea is also one of six recent guilty pleas Manhattan prosecutors have negotiated as investigators continue their ongoing probe into the growing number of city tax cheats using Internet sites, offshore banks, and shell companies to conceal assets and wealth.

Once thought of as a tax-shielding mechanism used only by masters of high finance, the practice of laundering money through offshore sites has become increasingly mainstream, middle-class, and cost-effective, said Daniel Castleman, chief of investigations at the district attorney’s office.

In order to learn how to disguise money in international bank accounts, John first attended a conference on the tropic island of Antigua that offered instruction in the creative arts of offshore accounting.

Last year, the Internal Revenue Service identified at least 400 individual promoters that specialized in offering services to aid those wishing to shelter taxes.

After returning from Antigua, John began a relatively simple scam, prosecutors said, billing his foot shops for bogus business expenses, and then sending the payments for those bogus bills to shell companies he controlled in the Bahamas and Barbados.

The foot doctor then began to withdraw the hidden income through local ATM machines and by using corporate credit cards – for as much as $20,000 in one two-week period last year.

“With the advent of some Internet sites, we’re finding that money-laundering has become much easier,” Mr. Castleman said. “It’s not just for tax cheats anymore.”

Investigators first stumbled upon John’s mischievous transactions about 16 months ago after slapping the MasterCard credit card company with a subpoena demanding information.

They were looking for transactions made by tri-state residents in international locations where offshore banks are prevalent.

When the results of the subpoena came, Mr. Morgenthau said he was “shocked.” There were 115,000 separate accounts in the tri-state alone.

While the account of a city tourist using a credit card to buy a souvenir T-shirt in the Bahamas might have popped up in their results, Mr. Castleman said that if only half of MasterCard’s account holders held offshore accounts the net amount of taxes lost would still be as chilling to collectors.

The district attorney’s office has 40 active cases open against tax dodgers using offshore sites. Investigators said they continue the laborious task of combing through 115,000 of the MasterCard accounts in hopes of finding offshore transactions that don’t add up.

“We’re identifying and isolating,” Mr. Castleman said.