The ringleader of the heist gang behind one of the biggest burglaries in English legal history has been ordered to pay back nearly ?6 million or serve an extra seven years in jail, police said on Thursday.

Michael Seed, 60, known as "Basil", was jailed for 10 years in March 2019 over the 2015 Hatton Garden jewellery raid in the capital's diamond district.

It netted ?13.7 million ($18.2 million) of gold, jewellery and precious stones.

A proceeds of crime hearing held in July has ruled that Seed must pay back ?5,997,664 ($7.7 million) within three months or see his sentence extended, a police statement said.

The alarm specialist evaded capture for three years following the daring burglary, in which the six-man gang used a diamond-tipped industrial drill to bore through a concrete vault wall over a long holiday weekend.

Detective Chief Inspector Mark Bedford of London's Met Police said that to date just over a third of the property stolen in the Hatton Garden raid had been recovered and much of it returned to the victims.

When police raided Seed's flat, he  was found breaking up gold ingots on a workbench in his bedroom
When police raided Seed's flat, he was found breaking up gold ingots on a workbench in his bedroom METROPOLITAN POLICE / HO

"Crime must not pay, and this should demonstrate the lengths we will go to ensure criminals are prevented from enjoying any financial benefit from criminality," he said in a statement.

Seed was arrested at his flat in north London in 2018. Police found 933 items of jewellery stolen in the raid, including gold ingots he was breaking up on a workbench in his bedroom.

Alongside a smelting machine, police also found burglar alarm technical manuals and signal blockers for alarms.

Seed, who at the time of his arrest paid no taxes, claimed no benefits and rarely used a bank account, claimed he was an amateur jeweller who came by the stolen items innocently.

But a jury later found him guilty of conspiracy to burgle and of converting the proceeds of crime.

The audacious raid has since inspired several movies, including "King of Thieves" starring Michael Caine.

Prior to Seed, nine people had been convicted and imprisoned for crimes related to the heist, receiving sentences ranging from 18 months to seven years.