Goldsmith’s team said the strikes planned for this weekend would be illegal under the government’s Trade Union Bill – a measure Khan opposed.
The planned strike is based on a strike ballot held last June, and would not be allowed to go ahead under the government bill, which imposes a new four-month time limit on ballots.
Tory MP Bob Neill said: “The cost of strikes is crippling, up to £300m a day, and the cost of the Khan-Corbyn experiment is clear: by opposing new strike laws and bringing the unions into the heart of City Hall strikes would go up.
A Khan spokesperson rejected the claims, saying: “Sadiq does not support the strike this weekend and has said the RMT should call it off. Official statistics prove that Londoners who want fewer strikes should vote for a Labour mayor. The number of Tube strikes has more than doubled since Boris Johnson became mayor and it would be exactly the same under Zac Goldsmith.”