A state general assembly approved a measure to ban capital punishment, which would make the state the 18th in the US to do so.

Maryland governor Martin O'Malley is expected to sign the ban into law.

Supporters of the ban argue that capital punishment is costly, error-prone, racially biased and a poor deterrent.

Opponents say it is a necessary tool to punish those who commit the most serious crimes.

Maryland has five men on death row, though the measure makes it clear the governor can commute their sentences to life in prison. The state's last execution took place in 2005.

Capital punishment has been on hold in Maryland since a December 2006 ruling by the state's highest court that the lethal injection protocols were not properly approved by a legislative committee.

Maryland has a large Catholic population, and the church opposes the death penalty.

According to the Maryland Department of Public Safety and Correctional Services website, Maryland has only executed five inmates since 1976.

In contrast, neighbouring Virginia has executed 110 inmates since the US Supreme Court restored capital punishment in 1976, according to the Death Penalty Information Centre.

However, Virginia's death row population has dwindled to eight from a peak of 57 in 1995, in part because fewer death sentences are being handed down in the state amid an increased acceptance of life without parole as a reasonable alternative.

The centre said death sentences have declined by 75% and executions by 60% nationally since the 1990s.