The new probe will expose the 54-year-old Dublin criminal as the beneficial owner of 13 properties in the Mediterranean country, despite being under tight security in the Co Laois jail.

In the first major expose of the infamous criminal since he was convicted of drugs offences five years ago, RTE'sPrime Time Investigateswill show that Gilligan also remains a major force in the drugs business to this day.

In a documentary to be screened tomorrow evening, the programme will reveal that Gilligan is involved in organising and controlling a massive international criminal network from behind bars.

The programme includes a strongly-worded reply by the drug gangster to the claims that he remains a major underworld figure. When contacted by Prime Time, Gilligan insisted: "None of my money is wrongly got. None of my property is wrongly got. I can account for that when I have to, and when I'm good and f**king ready, I'll account for that."

Asked about her alleged role as a "godmother" of crime, Gilligan's wife Geraldine also mounts a robust defence: "That is bollox. I am not the godmother of crime. I have never been anywhere near crime, " she insists. Geraldine Gilligan is now based in Spain.

The programme will reveal that Gilligan organised the property investments while he was awaiting trial for Veronica Guerin's murder. He was acquitted in relation to the homicide charge but was subsequently convicted of running a drugs empire.

Drug dealer exodus There has been a dramatic exodus of Irish drug dealers to the continent in recent years. The Sunday Tribune revealed last year that gardai and Spanish police have identified a large number of Irish criminals, particularly from the west Dublin area, who have moved to the Alicante region in recent years. They are concentrated in particular on the Torrevieja area of Alicante, where legitimate Irish property investors are also a major force.

Two- to three-bed townhouses in the Spanish city can cost from around 140,000 upwards, while small apartment are much cheaper than in Ireland, at around 85,000 for a 75sq ft apartment.

The Sunday Tribune investigation found at the time that while there is no clear indication that a large amount of illegal drugs business is being conducted by Irish criminals in the city. The booming property trade has also seen an influx of Russian mafia, as well as Moroccan and expatriate English drug dealers, all of whom are eager to expand traffic routes for the transit of hard drugs, especially cocaine, into mainland Europe, the UK and Ireland.

The RTE investigation also explores the dealings of George 'The Penguin' Mitchell. The programme makers dramatically filmed Mitchell in action as the veteran criminal walked from his Dutch home to make calls from public phone boxes in the apparent belief that it was a safe way to communicate. Mitchell is regarded as a major international crime figure by police. Gardai interviewed on the programme express serious concern about the apparent ease with which Mitchell operates in Holland.

But it will be the programme's focus on John Gilligan that will fascinate most, particularly given the massive cost to the exchequer of keeping the drug dealer behind bars, as well as the legal battle which the Criminal Assets Bureau (CAB) has fought to get their hands on some of the criminal's Irish-based assets.

Drugs gang At Gilligan's 2001 trial on drug-related charges, the Special Criminal Court was told that Gilligan controlled a group of criminals, named as the Greenmount Gang, which was involved in the shipment and distribution of massive quantities of illegal drugs. For this he received a 28-year sentence, which was later reduced to 20 years.

When gardai raided his property, they recovered a large consignment of cannabis resin and lists indicating the illegal transit of significant amounts of the drug from overseas destinations.

Originally from Dublin city centre, Gilligan formed his gang in the early 1990s and initially developed his criminal enterprise largely away from garda attention, under the cover of his involvement in apparently legitimate businesses.

Guerin murder He had previously been known to gardai as a petty thief and had been involved in robberies on warehouses and goods. However, he developed a large-scale cannabis-selling enterprise with the assistance of several well-known criminals, including his lieutenant, Brian Meehan. Gilligan's gang first came to the attention of Veronica Guerin when he was being probed by the garda drugs squad over suspicion that he was becoming one of the dominant forces in the Dublin drugs trade.

Asst Garda Commissioner Tony Hickey told the Special Criminal Court in 2001 that the net profit from drugs sales organised by Gilligan was over 17m. Hickey said that over 2,000kg was imported from Amsterdam through Cork. Hickey told the court that there was "no evidence whatsoever" that anyone other than Gilligan benefited from the 2,800 per kg he received for the cannabis.

Chief Supt Felix McKenna of the CAB also told the court that it had only identified "a small amount" of Gilligan's assets at that stage. He said Gilligan had spent over 2m on Jessbrook Equestrian Centre alone.

In an interview with the Sunday Tribune last year, prior to his release from prison, former Gilligan gang member Patrick 'Dutchy' Holland claimed that a member of Gilligan's outfit, Charlie Bowden, was Guerin's killer. Bowden, an ex-army marksman, admitted loading the murder weapon. Bowden was one of the state's main witnesses in the case against Gilligan and is now believed to be living overseas under the witness protection programme.

Figures from the prison service show that it will cost the state over 5m to keep Gilligan in the highsecurity wing of Portlaoise prison over the term of his sentence; costing 230,000 a year to keep the major criminal in Portlaoise.