Westminster council had been due to sign up to the TfL year-long trial – the only legal way to ride an e-scooter on public roads and cycle lanes in the capital.
But the Tory-run borough was not included alongside the City of London, Lambeth and Southwark who on Monday became the latest participants in the Government-backed scheme.
TfL sources said it was hoped that Westminster would now join the trial next month. The council confirmed it hoped to join when the scheme next expands on August 2.
A council spokesperson said: "Westminster City Council is preparing to join the next phase of the Transport for London e-scooter trial and we will make an announcement shortly.
“We have identified appropriate parking locations for this scheme and we are monitoring issues from the roll-out in other boroughs to ensure the trial in Westminster runs as smoothly and safely as possible.”
Tower Hamlets also took joined as a “ride through” borough, meaning the battery-powered scooters can be ridden on its streets but not hired or left on council land.
But hopes that the City of London would be a launch borough – enabling trips between the financial districts of Canary Wharf and the Square Mile - were dashed.
The scheme did expand to the City on Monday, with the Square Mile and parts of north Lambeth becoming full participants, and Southwark becoming a “ride through” borough.
It means the total number of e-scooters available for hire in London will double to 1,200.
The scooters, which are limited to 12.5mph, have on-board computer systems that use GPS “geo-fencing” to bring the rider to a halt if used outside of approved areas.
Three operators – Dott, Lime and Tier – are taking part. It costs between £3.25 and £3.40 for a typical 15-minute ride.
TfL and London Councils are continuing discussions with boroughs about further expansions over the coming months. The Royal Parks has opted out of the trial, meaning the scooters cannot be used in areas such as Richmond Park and Hyde Park.
Will Norman, London’s walking and cycling commissioner, said: “Extending the footprint of the trial to more areas will enable us to get a better understanding of the role e-scooters can play in switching car journeys to greener and more sustainable alternatives.”
Concerns were expressed after last month’s launch that the silent nature of the e-scooters presented a risk to blind and partially-sighted Londoners.
TfL said that each operator is working on adding an “appropriate sound” to their scooters to alert pedestrians to their presence, with Tier committed to upgrading its vehicles later this summer.