The bank suggested that as many as 40,000 City workers could be laid off due to the ongoing credit crisis.
This doubles a previous estimate from the UK's Centre for Economics and Business Research - and includes around five per cent of all jobs in the sector.
By way of comparison, losses of seven per cent were registered in the wake of the dotcom boom at the beginning of the decade.
"We expect…minus 16 per cent rental growth [in the City]," the JP Morgan note added.
The annual contribution of the financial services industry to the UK economy currently stands at around £100 billion, the Daily Telegraph reports.
Recently, Scottish bank RBS announced the loss of 200 jobs - with similar retrenchment statements from UBS and Merrill Lynch expected soon.