Chrishanthakumar had been warned by UK authorities, the court heard
The founder of the British Tamil Association has been convicted of two terrorism charges.
But the jury was discharged after failing to reach a verdict on three other charges against Arunachalam Chrishanthakumar, 52, of south London.
He was found guilty of supplying bomb-making equipment for the Tamil Tigers and receiving documents for the purpose of terrorism.
The Tamil Tigers became a banned organisation in the UK in 2000.
The jury at Kingston Crown Court was also unable to reach a verdict on one charge against co-accused Jegatheeswaran Muraleetharan, or Muralee, of Powys, Wales.
He had denied receiving electrical components for the purposes of terrorism.
Chrishanthakumar, known as Shantham, is a property valuer from Norbury, in south London, and the founder of the British Tamil Association.
Born in Sri Lanka, he was head of the United Tamil Organisation in the UK until it was proscribed in 2001.
The court was told he had been warned by the UK authorities in 2004 after he bought boots and handcuffs for the Tigers' police force.
Prosecutors said that despite this, he continued supplying electrical components for the Tamil Tigers, including some which had "an obvious terrorist purpose".
Thousands marched through London in protest against the war in Sri Lanka
A majority of jurors decided that he was guilty of receiving electrical components as well as literature and manuals with titles such as Underwater Warfare Systems, Explosive Ordnance Disposal and Naval Weapon Systems.
But the jury of five men and seven women was unable to reach a verdict on three other charges against him.
These were: receiving military equipment, receiving money, and belonging to a proscribed organisation, all of which Chrishanthakumar denied.
During deliberations, the jury sent almost 70 notes to Mr Justice John Saunders.
One member asked to be discharged because deliberations were proving "too stressful".
The Crown Prosecution Service will decide next week whether there will be a re-trial.
On 6 April another co-accused, Murugesu Jegatheeswaran, 34, of Mitcham, south London, was cleared of receiving electronic items for use in terrorism.
Mr Muraleetharan's brother, Jeyatheswaran Vythyatharan, or Vithy, 40, of Powys, Wales, was also cleared of the same charge following the nine-week trial.
The Tamil Tigers, or Liberation Tigers of Tamil Eelam (LTTE), have waged an armed campaign for more than 30 years to try to create an independent Tamil state on the island of Sri Lanka.
There are 300,000 Tamils living in the UK, half of them in London.
On 11 April 100,000 protesters marched through the city calling for an end to war in Sri Lanka.