Image copyright PA Image caption John "Goldfinger" Palmer was jailed in 2001 for a £20m timeshare fraud

A notorious conman who was shot dead last year had been under surveillance by secret intelligence officers for at least 16 years, the BBC has learned.

John "Goldfinger" Palmer, who was once described as Britain's richest criminal, was shot dead in the garden of his home in Essex in June 2015.

An operation, codenamed Alpine, had gathered intelligence on Palmer from RAF Spadeadam in Cumbria since 1999.

Police said Palmer's killing had "all the hallmarks" of a professional hit.

Media playback is unsupported on your device Media caption A former intelligence officer says Palmer was under surveillance for 16 years until his death last year

Palmer, a former scrap metal dealer from Olton, near Birmingham, got his Goldfinger nickname after bullion from the the 1983 £26m Brink's-Mat robbery was melted down in a shed in the grounds of his country house at Lansdown near Bath.

He was jailed in 2001 for a £20m timeshare fraud in Tenerife, and at the time of his death, at the age of 65, he was due to stand trial in Spain on charges of fraud, firearm possession and money laundering.

Shortly after his death it was revealed he had been under surveillance since 2007, but the BBC has discovered the Cumbria team had been monitoring Palmer since 1999, including the four years he spent in jail.

Image copyright PA Image caption Palmer's "Goldfinger" nickname derived from his connection to the 1983 Brink's-Mat gold bullion robbery

A former intelligence officer, who did not wish to be identified, said the the Serious and Organised Crime Agency (Soca) - now the National Crime Agency - had gathered intelligence on Palmer from Cumbria because of concerns of corruption in the Metropolitan Police Service.

"The team there then collated the info together, and some of that intel was then used to inform police investigations into various parts of his criminal organisation," the officer said.

"We knew what police officers he had in his pocket, what other criminals he was mixing with, and using for various criminal enterprises."

Earlier this year a £50,000 reward was offered to help catch Palmer's killer, and the case featured on BBC One's Crimewatch, but the gunman has not been caught.

The reward is offered in part by his partner Christina Ketley, who was convicted in 2001 for being involved in Palmer's timeshare fraud and who is facing separate charges in Spain - accusations she denies.

BBC Inside Out's special programme on John Palmer, presented by veteran broadcaster Roger Cook, can be seen on BBC One in the West at 19:30 BST on Monday 26 September, and afterwards on the BBC iPlayer.