It is thought the works, for the £15 billion Crossrail project, will eventually yield up to 4,000 bodies, some dating back 450 years.
The graveyard, built on the Bethlem Hospital’s vegetable patch in the 1560s after churchyards around the city started to overflow, was used to bury London’s poor and religious non-conformists as well as inmates from the asylum.
Conditions inside Bedlam were depicted by William Hogarth in his 18th century drawings A Rake’s Progress, charting the decline of a merchant’s son from wealthy heir to asylum inmate, via debtor’s jail. In 1676, the asylum moved to nearby Moorfields, then in 1815 to Southwark. Now the Bethlem Royal Hospital, it has been in Beckenham, Kent, since 1930 and is part of the South London and Maudsley NHS Trust.