PORTLAND – A longtime partner at a prestigious Portland law firm pleaded guilty Tuesday to stealing nearly $300,000 from clients and the firm. The plea was part of an agreement in which the state recommended that he serve 21/2 years in prison.

John Duncan, who was fired last year by Verrill Dana after the theft came to light, appeared in Cumberland County Superior Court.

The 54-year-old Falmouth resident admitted stealing about $109,000 from clients and $187,000 from the firm over a 10-year period. Duncan had been with Verrill Dana for 29 years, specializing in wills, trusts and estates.

Assistant Attorney General Mike Colleran recommended a six-year sentence, with 31/2 years suspended, along with three years of probation and restitution of $287,000.

Justice Robert Crowley deferred sentencing until Duncan appears in federal court on tax-related charges. His lawyer, Toby Dilworth, told the judge that his client will plead guilty to those charges sometime in the next three months.

The theft was detected after a legal secretary reported that Duncan had written several checks to himself from the account of an elderly client. A subsequent audit uncovered other irregularities.

Duncan, who has submitted a letter of resignation from the bar, also faces professional discipline. The state board that oversees lawyers’ conduct has passed the case to the Maine Supreme Judicial Court.