The Provisional IRA has formed alliances with criminal gangs in the rest of the UK to distribute smuggled cigarettes and illegal fuel, BBC News has learned.
Illegal fuel is thought to generate millions of pounds
An investigation by Radio 4's File on 4 programme found the IRA to be behind the smuggling of millions of pounds of contraband across the Irish Sea.
It says for international operations, the IRA uses structures put in place to import arms during the Troubles.
British criminal gangs are used as its
distribution arm in the rest of the UK.
The UK is a lucrative market for smuggled cigarettes because of high levels of taxation.
Bill Lowry, the former head of Special Branch in Belfast, told File on 4 that alliances between the IRA and criminal gangs were formed in prison.
Mr Lowry told the programme "a very, very good network" was in place which enabled cigarettes to be brought in and diesel to be smuggled.
'Friends in jail'
"They have a network within the criminal community in mainland UK, a number of them did a lot of jail time in the high security jails and made good friends, so they have contacts that are out there," he said.
Last month, customs officers at Dublin docks seized lorry trailers disguised as general cargo vehicles but converted into fuel tankers, bound for Liverpool.
Irish customs officials linked this with another operation they uncovered last year, and believe that nearly five million litres of laundered fuel has been sent from Dublin Port across the Irish Sea and into Britain by this one group alone.
File on 4 reports that such rackets are making fuel launderers and cigarette smugglers millions of pounds of profit through unpaid taxes.
An Organised Crime Task Force has been set up in Northern Ireland to co-ordinate the efforts of the law enforcement and revenue agencies trying to combat this problem.
Its latest threat assessment says the Provisional IRA is deeply involved in these kinds of activities, as are some of the other gangs with paramilitary associations in Northern Ireland.
The government says its multi-agency enforcement approach is beginning to have an impact on Northern Ireland's serious organised crime problem.
But Security Minister Ian Pearson told File on 4 the Provisional IRA is a "highly disciplined organisation".
"They are perhaps the most sophisticated organised crime grouping to be found, possibly anywhere in the world," he said.
"This is not going to go away. There is no sudden thing that any government anywhere in the world can do that will mean that organised crime disappears. It has been a long battle."
File On 4: BBC Radio 4, Tuesday 15 March, 2005 at 2000 GMT, and repeated on Sunday 20 March, 2005 at 1700 GMT.