Poor transport is damaging London's reputation as a world business capital, according to a survey.
Skills shortages and high crime rates were also highlighted as undermining London's image, in the Confederation of British Industry (CBI) poll.
Sir Digby Jones of the CBI warned against complacency in its "continuing attractiveness" for businesses.
He called on politicians to ensure they did not "add to the burdens that will tip the balance the wrong way".
Expensive city
Sir Digby said: "It's clear business people make some finely balanced judgments when they assess whether the pluses of being in London outweigh the minuses."
The survey also revealed that most firms think London is more expensive than other capital cities, but that it is still a good place to do business.
He said: "For many that balance remains in London's favour, but no-one should be complacent.
"Politicians must work at securing London's place as a world business city."