Daniel Jones, 59, from Enfield, is currently serving a prison sentence for his role in the infamous £25m jewellery heist of 2015.
He has now admitted being involved in an attempt to break into a safe in Chatila jewellers in Old Bond Street, Mayfair, in August 2010.
Two other men have pleaded not guilty.
Terry Perkins, 68, of Enfield, has denied making off with goods worth more than a million US dollars from a jewellers between 27 and 31 August 2010.
Charles Matthews, 54, from Virginia Water, Surrey, denies one count of handling stolen goods.
They are both due to stand trial on 27 February at Southwark Crown Court.

The foreign ministry said it acknowledged a "miscommunication" with Iraq over the deployment of its forces.
It comes a day after US President Barack Obama urged Turkish President Recep Tayyip Erdogan to "de-escalate tensions" with Baghdad.
Turkey sent at least 150 troops into northern Iraq earlier this month saying they were to protect military trainers.
Turkey has deployed troops at the Bashiqa camp near Mosul - a city controlled by Islamic State (IS) - since 2014 to train Iraqi Kurdish forces.
But the Iraqi government said the latest move had been carried out without consultation and violated national sovereignty and international law.
"Turkey... acknowledges the miscommunication with the government of Iraq over the deployments of Turkish protection forces," the Turkish foreign ministry said in the statement.
"Turkey, in recognition of the Iraqi concerns and in accordance with the requirements of the fight against Daesh [IS], is continuing to move military forces from Nineveh province that were the source of miscommunication."
The statement did not say how many troops would be moved or where they would go to.
On Monday, Turkey's state-run news agency quoted military officials as saying a 10 or 12-vehicle convoy had left Bashiqa camp and was moving north.
It came days after Turkish Prime Minister Ahmed Davutoglu's office said it had decided to reorganise its military personnel at Bashiqa following talks with Iraqi officials.
The Turkish government enjoys close relations with the semi-autonomous Kurdistan Region in Iraq.
But it considers the Syrian Kurdish Democratic Union Party (PYD) - whose Popular Protection Units (YPG) militia is a key ally of the US-led coalition against IS - as an enemy because it is an offshoot of the banned Kurdistan Workers' Party (PKK).
One of the ringleaders behind the Hatton Garden raid has admitted trying to steal £1m of gems from a London jewellers five years earlier.
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Turkey says it will pull more of its troops out of northern Iraq, following a partial withdrawal earlier this week.