More than half of London's emergency service workers live outside the capital, raising concerns about responses to major incidents, the London Chamber of Commerce (LCC) says.
Just over half of police, fire and ambulance staff commute to London for their shifts, it said.
NHS wages rose by up to 4% in recent years but travel and housing costs in London increased by about a third.
LCC chief executive Colin Stanbridge said its findings were concerning.
"The report has found that a majority of London's main 'blue light' emergency services workers live outside London.
"As London moves towards becoming a 'megacity', the preparedness of 'blue light' emergency services is a matter that the new mayor - and the new Assembly - will want to comprehensively examine."