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.
One of the ringleaders behind the Hatton Garden raid has admitted trying to steal £1m of gems from a London jewellers five years earlier.