A power cut set back the start of counting at Alexandra Palace – which is handling ballots cast in Enfield and Haringey; Barnet and Camden; Brent and Harrow; and North East London – by an hour.
London Elects spokesman Euan Holloway said: “It delayed the start of the count, but we don’t think it’s disrupted the progress of the count too much.”
He added: “The cause of the power outage is being investigated. There have been no issues at any of the other sites.”
It is believed the electronic process at Alexandra Palace was delayed after contractors working on the restoration of the building were not told of the count and cut off power.
Results are not expected to be officially announced until after 1800 BST.
The mayoral contest outcome may not be announced until after 2000 BST if, as expected, no one candidate gets more than 50% of first preferences.
If that happens all the candidates apart from the top two – Mr Johnson and Mr Livingstone – will be eliminated, with their second preference votes being reallocated to the top two.
These reallocated votes will be added to their first preference totals, with the winner being the man with most votes.