Sadiq Khan has knocked back calls for three green ferry crossings to be created in east London, arguing that there is not currently enough money to pay for them.
The mayor faced criticism from City Hall Conservatives, who said he was instead choosing to "fund his pet projects and further his own political interests".
Mr Khan recently announced a one-year freeze for most Tube and bus fares, and revealed on Sunday that he will scrap peak fares from the Tube on Fridays for three months from March.
The request for the mayor to introduce the new electric ferry crossings - at three key locations east of Canary Wharf - was made by the Port of London Authority (PLA) in partnership with the Thames Estuary Growth Board (TEGB).
The two organisations commissioned a study by the consultancy Arup, which found that delivering crossings between the Isle of Dogs and North Greenwich, Royal Wharf and Charlton, and Barking Riverside and Thamesmead, would bring significant benefits by connecting some of London’s fastest-growing areas.
If a £1 fare was set for crossing the river, the cost of establishing and running the three proposed ferries for a period of 30 years is estimated to be £370m in total.
The study was published at the end of last year, and was raised earlier this month in a written question to Mr Khan by Tory assembly member Nick Rogers.
Asked by Mr Rogers whether he supported the plans, the mayor’s office said: "Transport for London (TfL) regularly reviews new river crossing proposals for London.
"However, there remains significant uncertainty around TfL’s long-term capital funding. While TfL continues to progress committed schemes and agreed investments with [the] Government, TfL is not in a position to invest any significant capital costs in new schemes such as the aforementioned new ferry crossings.
"TfL remains supportive of the principle of new river crossings and is happy to support the concept in any way possible within current funding limitations. TfL is also interested to see progress with Uber Boat by Thames Clippers’ plans for an improved cross-river ferry service between Rotherhithe and Canary Wharf."
Mr Rogers told the Standard: "In the past few weeks, Sadiq Khan has found £177m for desperate pre-election giveaways - £30m to stave off Tube strikes, £123m to freeze TfL fares for one year and £24m to fund a temporary three-day weekend on the Tube by pausing peak fares on Fridays.
"Nobody believes Sadiq Khan anymore when he says there’s no money. Sadiq Khan finds money to fund his pet projects and further his own political interests. He chooses not to fund projects that truly matter to Londoners, such as introducing new fast, green ferry crossings across the Thames."
Mr Rogers added that the creation of crossings in the areas identified in the PLA and TEGB’s report "would bring huge benefits for residents and businesses".
"If Sadiq Khan is not prepared to progress these new routes, he should step aside for someone who will," he said.
The Labour mayor has previously emphasised his support for more river crossings in east London, arguing that the area has been "neglected for centuries" in terms of its cross-Thames connectivity.
He has said that this inequality between west and east London is a key reason why he supports the construction of the Silvertown Tunnel between the Greenwich Peninsula and Royal Docks. The new road tunnel is due to open in 2025.
From 2028, TfL hopes to begin construction on a planned cross-river extension to the DLR, from Gallions Reach to Thamesmead via Beckton Riverside. The service could be open to passengers from the early 2030s.
