(Reuters) - U.S. Representative Michael Grimm of New York said he would not resign from Congress following his guilty plea on Tuesday to a federal felony tax charge.

"As long as I'm able to serve, I'm going to," said Grimm, who noted he easily won a third term in November despite a 20-count federal indictment unveiled in April.

Grimm, a Republican, pleaded guilty in Brooklyn federal court to aiding the preparation of a false tax return in connection with a health food restaurant, Healthalicious, that he co-owned before his political career.

"While operating a restaurant, we underestimated the gross receipts and used some of the money to pay employees off the books and some other expenses," Grimm said in court.

As part of a plea deal, Grimm, whose trial had been scheduled for February, also signed a statement of facts, admitting to concealing over $900,000 in gross receipts from 2007 to 2010 and lying during a 2013 deposition.

Grimm's defiant declaration that he will not resign could put U.S. House of Representatives Speaker John Boehner and other Republican leaders in a difficult position as Democratic members have already begun calling for Grimm's resignation.

"Clearly, Speaker Boehner must insist that Congressman Grimm resign immediately,” said House Democratic leader Nancy Pelosi in a statement from her home earlier on Tuesday.

A Boehner spokesman said he would not comment until he has discussed the issue with Grimm. Grimm told reporters he has had "private discussions" with leadership but would not elaborate.

The 44-year-old former Marine and FBI agent, who represents Staten Island and parts of Brooklyn in New York City, faces a maximum of three years in prison when he is sentenced on June 8. His lawyers indicated they would seek a more lenient sentence.

Grimm told reporters he was accepting responsibility for a "mistake" that occurred before he joined Congress.

"For the past four years, I've been a very effective, strong member of Congress," he said, adding that he had received many words of support from his constituents.

House members who plead guilty to a crime that carries two or more years in prison "should" refrain from voting on the floor or participating in committee business, according to House rules.

The House could also vote to reprimand, censure or even expel Grimm, as it did in the case of Democratic Representative James Traficant, who was found guilty of taking bribes and other crimes in 2002 but refused to resign.

Prosecutors had accused Grimm of hiring illegal immigrants, paying staffers under the table and under-reporting how much he spent in wages.

He was also charged with lying under oath about his practices while defending against a lawsuit brought by former Healthalicious employees.

The indictment grew out of a probe of Grimm's fundraising, morphing into one of the highest-profile prosecutions by the office of Brooklyn U.S. Attorney Loretta Lynch, who has since been nominated to become U.S. attorney general.

Lynch in a statement said the plea made clear that U.S. authorities "will vigorously investigate and prosecute fraud wherever we find it, and that no one is above the law."

Grimm had already generated controversy in January, when he was recorded on camera threatening to toss a reporter off a balcony and break him "in half, like a boy." He subsequently apologized.