The veterans of Britain's Parachute Regiment -- made anonymous by court order -- were summoned to Belfast Crown Court where they denied the murder of Joe McCann in 1972.

The 24 year-old Official Irish Republican Army member was shot in the Markets area of Belfast at the height of three decades of violence over British rule of Northern Ireland.

The trial, which is due to last four weeks, has angered the serving and former members of the military, and seen the government vow to legislate to prevent further prosecutions.

Opening the case, a prosecution lawyer said McCann was a senior member of the Official IRA who was suspected of involvement in a number of attacks. He said McCann was shot in the back as he ran away after evading arrest.

"On any view of the facts the level of force used was unreasonable," he told the court.

A lawyer representing one of the soldiers said McCann was suspected of involvement in murders. She insisted the force used was reasonable and said the men faced a "binary choice" of firing to effect an arrest, or letting McCann escape.

The court heard that Mr McCann sustained three bullet wounds, two of which may have been caused by one round.

One wound was fatal but prosecutors contend both soldiers are responsible for the murder, regardless of who fired that shot.

In statements given in 2010 both soldiers insisted they used "reasonable force in all circumstances".

Six former military personnel have been charged with offences relating to "The Troubles", according to a UK parliament briefing paper published in February.

Prosecutions include Bloody Sunday in 1972, when soldiers opened fire on a peaceful civil rights march in the Bogside area of Derry, killing 14.

Prosecutions from the era of "The Troubles" are freighted with controversy in Northern Ireland, which remains split along sectarian lines despite a 1998 peace deal.

British servicemen arrived in the province on a mission to keep the peace in 1969 but were involved in some of the bloodiest chapters of the conflict, which saw a total of 3,500 killed on all sides.

Some feel soldiers' actions were state-sanctioned and legitimate by comparison to shadowy paramilitaries operating amongst pro-Ireland nationalist and pro-UK unionist communities.

Others feel servicemen should be held to higher standards than paramilitaries and that a blanket amnesty would imply guilt amongst all soldiers.

According to Ulster University's Sutton Index of deaths, the British Army was responsible for around 300 killings over the course of operations, which officially ended in 2007.

British Prime Minister Boris Johnson's government has pledged to protect former fighters from "vexatious claims" in a deeply controversial campaign promise which riled the Irish government.

Veterans minister Johnny Mercer resigned his post last week, voicing frustration about a lack of progress in legislating that promise.

He appeared outside court in Belfast on Monday to offer his support to the ex-soldiers.

"I think it is unfair to try and apply today's standards of operations and retrospectively apply them to that time and try to get justice," he told reporters.

"They served their country, they did their best. War is messy and we need to find a solution for everybody."