Sadiq Khan has been accused of “misleading” voters by exaggerating potential savings from moving London’s government out of City Hall.
Conservative Party analysis claimed a proposed move to The Crystal building in Newham would save £5.6m a year.
The Mayor of London promised moving out of City Hall, near Tower Bridge, would save £11.1m a year in rent and charges.
The mayor’s office said the figure was calculated by professionally-qualified finance officers.
A spokesperson for the Mayor of London said: “The proposed move to the Crystal Building will save the Greater London Authority (GLA) Group £55m over five years.
“The move is only necessary because the government is not adequately funding local and regional government in London for the cost of tackling Covid-19.”
Norman Foster-designed City Hall has been the official home of the GLA since it opened in 2002.
Under the plans the mayor’s office and London Assembly would move to the GLA-owned The Crystal in the Royal Docks, which was commissioned to be one of the most environmentally sustainable offices in the world.
The move would also see the GLA use office space at Palestra House at Blackfriars, currently used by Transport for London.
A formal six-week consultation on the move ended on 5 August.
The Conservatives said Mr Khan failed to include potential lost income from leasing The Crystal and Palestra to private renters in his announcement.
If the GLA were to stay put, letting these spaces could generate £4.7m a year, according to the analysis.
The Conservatives said Mr Khan also failed to include the £280,000 a year generated by public events held at City Hall and the Crystal under the current set-up.
This would halve total savings over five years from £55m to £27.76m, the party said.
But the mayor’s office said “significantly lower running costs” at the new sites would be expected “to offset any hypothetical loss of income from renting out The Crystal”.
Susan Hall, Conservative leader on the London Assembly, said: “The mayor is misleading Londoners.
“We’ve calculated that the real savings figure could be less than £6m a year, which pales in comparison to the millions of pounds Mr Khan wastes each year.
“This is yet another example of the mayor putting PR before policy.
“I urge the mayor to go back to the drawing board and come up with an honest assessment of the cost of moving City Hall.”
The lease for City Hall was agreed with a private landlord, the Kuwaiti-owned St Martins Property Group, in 2001 and is due to run for 25 years.
But the agreement allows for a break in the contract after 20 years – in December 2021 – which will be the only chance the GLA has to leave early.