Concerns about dissident republican use of Semtex in a weekend attack continued to build yesterday, apparently under the impression that the presence of the deadly explosive marked a new departure for the breakaway IRA groups.

But Friday night's attempted attack in Fermanagh was not the first time the dissidents have used the Czech-made plastic explosive — it may not have been even the first time this year.

What’s crucial is the age of the Semtex being used - if it’s old Provo stock then it is nearing the end of its shelf life, which may be one reason it failed to explode in the weekend attack.

The Real IRA and the Continuity IRA have employed Semtex repeatedly since they broke away from the Provisionals in 1998.

It was used as the primer charge to detonate the Omagh bomb a decade ago and in varied incidents since then.

That didn’t stop the concern over its use spreading to political skirmishing between the DUP and their critics in Traditional Unionist Voice yesterday.

DUP MLA Ian Paisley Jnr called on dissidents to be "shot on sight" to "prevent dissident republicanism from growing" and said he will be making urgent inquiries about the use of Semtex through the Policing Board.

His comments came after TUV leader Jim Allister said the use of Semtex raised questions about IRA decommissioning.

"The people of Northern Ireland were assured the IRA had destroyed its Semtex, yet here it turns up ready to murder policemen in Lisnaskea," he said. "How can this be?"

Two policemen were treated for shock and minor injuries after dissidents tried to fire a homemade rocket launcher at them in Lisnaskea late on Friday night.

The device failed to go off and was subsequently found to contain Semtex.

On Monday Deputy Chief Constable Paul Leighton said its presence was "significant".

The Czech-made explosive was given to the IRA by Libya in the 1980s and tonnes were smuggled into Ireland.

The IRA prized Semtex because it is powerful, relatively easy to use and difficult to trace.

The explosive was supposed to be put "beyond use" in 2005 when the IRA completed decommissioning.

But by that stage dissidents were known to have seized some of the material from IRA arms dumps.

However, Semtex only remains effective for about 20 years.

Earlier this year Denis McAuley, the lead scientist at Forensic Science Northern Ireland, told the Omagh civil trial that Semtex was used as the booster charge to set off 150 to 200 kilos of homemade explosive in the 1998 Omagh car bomb.

It is believed Semtex was used as the primer in other massive car bombs set off by dissidents at that time.

And between 1999 and 2002 gardai Semtex seized from dissident republicans in Co Wexford, Gormanstown, Co Meath, Limericka and Dundalk.

In 2002, the PSNIfound it attached to under-car bombs in Londonderry.

Last year it was found in a mortar in Lurgan that had been set up to attack police.

A Real IRA statement also said Semtex was used in an under-car bomb attack on a policeman in Castlederg in May this year, although other reports suggested that the officer survived the attack because the bomb did not contain any significant high explosive.