HAMPDEN, Maine — A U.S. Postal Service sorting center employee has filed an unfair labor practice charge against his employer saying his April request to quit the union was filed late by a human resources worker and union dues continue to come out of his paycheck, officials confirm.

Brett Johnson of Holden filed the charge on Monday with Boston-based Region One of the National Labor Relations Board.

“A charge indeed was filed and it currently is under investigation by our office,” Elizabeth Gemperline, assistant to Regional Director Jonathan B. Kreisberg, said Friday.

She said she could not discuss details of the charge because of the ongoing investigation. Staff attorneys from the National Right to Work Legal Defense Foundation provided Johnson with free legal assistance and helped him file the paperwork, a press release states.

Johnson joined the National Association of Letter Carriers union in April 2003 and decided this year to withdraw from the union, the release states.

“On April 10, 2013, Johnson sent a letter to the NALC union resigning his membership and refraining from union dues payments,” it states. “Johnson hand-delivered the same letter to his postal service human resources representative the following day.”

Postal workers can opt out of the union, however, according to the union’s bargaining agreement, they only have a 10-day period — starting on April 10 each year — when they can withdraw and revoke their union dues deduction authorization.

“Despite his requests, the human resources representative failed to forward his letter to the proper human resources department officials for processing until June 10, 2013,” the press release states. “As a result, the USPS continues to deduct union dues from Johnson’s paychecks, and he cannot ask again to stop dues payments until next year.”

Johnson wants the union dues withdrawals to stop and a refund of all the dues collected since April.

“This case displays how union officials have the power to make it difficult for workers to exercise their right to refrain from paying union dues, even in situations where workers have Right to Work protections,” Mark Mix, National Right to Work Foundation president, said in the released statement.