When David Tomkins landed at Houston's George Bush International Airport two weeks ago, officials had no reason to suspect that the charming 63-year-old from Basingstoke was any different from his fellow passengers. Calmly handing over his passport, he gave the impression of a regular business traveller to Texas.

It all changed when an immigration official typed the passport number into a computer and armed guards came to arrest the British traveller. It is unlikely that even those passengers blessed with the most fertile of imaginations could have come close to guessing the extraordinary story that lay behind the arrest of the man from Hampshire.

This weekend Tomkins is locked up in a Florida jail, facing a five-year sentence. He is accused of masterminding a plot in 1991 to assassinate the Colombian drug baron, Pablo Escobar. More than a decade ago Tomkins was caught in an undercover sting operation by US Customs after he allegedly tried to buy a fighter jet to bomb a Colombian prison housing Escobar. The US authorities claim he has been on the run for the past nine years and have described him as a 'key player from the ranks of the international arms dealers'.

The story of how a man from a southern English commuter town became entangled in a plan to kill one of the most dangerous men in the world is an astonishing tale of illegal covert operations across three continents. It is also, as his wife, Mary, told The Observer this weekend, a tale of a man who had an 'almost unquenchable thirst for adventure'. 'Any chance he had of doing something exciting, however dangerous, he took it,' she said.

An investigation by The Observer has pieced together Tomkins's astonishing story. It shows how he graduated from being a professional thief to become a mercenary in a brutal conflict in Africa before becoming embroiled in a plot to kill one of the world's most notorious drug barons.

As a young man born with a taste for adventure, he soon specialised in blowing open safes in Britain and Europe. He once even broke into the safe of the Royal Military Academy Sandhurst.

After being in and out of jail for most of the 1960s he was offered the chance to fight as a mercenary during the 1970s, in one of Africa's bloodiest civil wars. With no job, no money and an addiction to an adrenalin rush, he left for northern Angola in 1976.

Once, when asked why he went, he said: 'It sounded like a good idea at the time ... [the man who offered me the job] could have said "Mars" and I would have said "yes". Within 48 hours I had a gun in my hand and I was in Angola.' Tomkins became one of the infamous 'Dogs of War' in Angola. He became the explosives expert under the notorious, bloodthirsty mercenary leader, 'Colonel Callan', described as a 'man of despotic power and Satanic terror'.

Tomkins became addicted to war. During the 1980s, this suave, blond-haired mercenary, who never went anywhere without his sunglasses and gold chain, had branched out into the arms trade, supplying weapons to various military forces. He was once asked to put together a team to assassinate the president of Togo and the Ugandan leader, Idi Amin, although neither contract came to anything.

It was not until the summer of 1988 that Tomkins took the call that would help seal his fate almost 15 years later. Through a contact in the arms trade, Tomkins was introduced to an officer from the Colombian army. He was asked to put together a team to destroy the headquarters of the FARC guerrilla group, which had been producing and smuggling cocaine.

At the time Colombia was in a state of unrest, with powerful rival drug gangs engaging in a fierce battle that was undermining attempts at stable government. Tomkins was offered $2,000 (£1,250) a week and a share of any loot found at the guerrillas' headquarters. He was told the operation against FARC was being funded by the notorious Colombian drug baron, Gonzalo Rodriguez Gacha - the head of the Cali drug cartel.

Tomkins then went about recruiting a team of mercenaries with a former colleague from the Angola campaign, the SAS officer, Peter McAleese. Tomkins told McAleese of his plans in the Booth Hall pub in Hereford.

McAleese described Tomkins as a man who had been wounded in Angola and showed 'a lot of guts'. Tomkins was a 'picture of studied elegance, tall, lean, tanned, his hair very grey and fashionably long'.

'I was amused to to see he had developed a habit of rather ostentatiously fingering various gold ornaments as he spoke; several large rings, a heavy bracelet and a medallion on the end of a thick, gold neck chain.'

The two mercenaries put together a team of 16, including two former SAS soldiers who took part in the 1980 Iranian embassy siege in London, and headed to Colombia. But the promised military equipment failed to turn up and the attack was abandoned. Tomkins returned to England in November.

Three months later, Tomkins received another call from his Colombian contacts. This time the offer was even harder to refuse: to attack and kill Pablo Escobar, the head of the Medellin drug cartel and the most infamous drug baron in Colombia, blamed by the US government for smuggling huge amounts of cocaine into the US.

Tomkins was assured by his contacts that there would be 'no government interference' in Colombia which would hinder his operations. This time he was to have an unlimited budget to buy the latest weapons and they were to be housed in a luxury villa above the town of Cali. Tomkins and McAleese assembled a team of 12 British mercenaries. The idea was to launch a helicopter attack on Escobar's 7,000-acre private estate, Hacienda Napoles, 80 miles east of Medellin, where the drug baron had a personal zoo full of exotic animals.

On 3 June Tomkins received orders to carry out the operation when Escobar was known to be at Hacienda Napoles. The flight from their basecamp in Cali to Escobar's estate was nearly three hours across heavy jungle.

When the helicopter Tomkins was flying in began crossing an 8,000ft mountain range, disaster struck: heavy rain clouds suddenly blocked the pilots' view and the helicopter flew into a peak. The pilot died but Tomkins and McAleese survived. It took them three days to walk out of the jungle and in June 1989 Tomkins called off the operation and returned home via Panama.

Despite such a setback, it appears that Tomkins could not rid himself of an ambition to kill Escobar, who had now become US public enemy number one. Even though Escobar knew what Tomkins was up to, the Basingstoke man once told a reporter in Colombia: 'Right now we have three choices; run, hide or go ahead. There is nowhere to run and we can't hide, so we are determined to go on until we finish what we came here to do.'

In 1991, a new project to assassinate the drug baron appears to have been put into place. US officials claim Tomkins was offered $10m to assassinate Escobar by a rival drug gang. Although Escobar had been caught by police he was being held in a specially built prison with plush accommodation and was thought to be still controlling elements of the lucrative cocaine trade.

This time, according to the US authorities, Tomkins planned to buy an A-37 Dragonfly light attack aircraft and bomb the prison. The plane, a Vietnam-era fighter jet, was armed with a machine gun in the nose and was capable of being armed with bombs. Tomkins was allegedly also trying to buy 500-pound bombs from El Salvador to use in the attack.

Unfortunately for Tomkins, US customs agents in Puerto Rico got wind of the plans and set up an undercover sting operation where their agents pretended to be willing arms suppliers. According to the indictment, in December 1991, Tomkins handed over $25,000 to undercover agents. But as he was about to be arrested, Tomkins received a phone call tipping him off about the sting. Tomkins fled back to Britain. In April 1994 the federal court in the Southern District of Florida indicted Tomkins for conspiracy to violate arms export laws.

For the past 12 years, his friends and family claim, he has not been involved in arms dealing or mercenary activity. They claim to have been 'shocked' at the arrest and allege that Tomkins had no knowledge of any charges he faced. They argue that he travelled to the US under his own name and was looking forward to using his 'expertise' to help the coalition forces in Iraq.

His wife explained how he was called by a friend who was working in Iraq for a US company, Skylink, which was recruiting security personnel. The firm had been given a contract by the US government to manage Iraq's three airports - in Baghdad, Basra and Mosul. She said he was flying to Texas to take part in a chemical weapons survival course at Fort Bliss.

She said: 'He believed this was his last chance for adventure and he could do something worthwhile. He believed his expertise would be very helpful to those helping to reconstruct Iraq.'

According to her, he was not scared by the worsening security situation. She said: 'He was aware of the dangers but that never frightened Dave. If he died in Iraq he would die happy. He was never the type man who I could see getting an illness and dying in a hospital bed.'

Tomkins's case is likely to come to trial early next year and he is believed to be determined to go to Iraq if he is cleared. It is not known what his defence will be, but what is certain is that whatever the outcome, another extraordinary chapter in the story of Tomkins's life will begin.