It means the number of e-scooters available to hire across the capital will increase to about 2,700 in the coming weeks – with the possibility that more boroughs will sign up for the year-long pilot.
It launched on June 7 in Canary Wharf in Tower Hamlets, plus Ealing, Hammersmith and Fulham, Kensington and Chelsea and Richmond.
Last month it was widened to the City of London, Lambeth and Southwark.
Monday’s expansion will allow the scooters – from private firms Dott, Tier and Lime – access to the City of Westminster for 10 months, with an option for a further six months.
However, as the Standard revealed last month, riders will not be able to hire or park them in Oxford Street or Regent Street.
They will be available for hire in nearby locations such as Soho Square, Newman Street, Stratford Place and Great Marlborough Street, as well as Trafalgar Square, Exhibition Road and Grosvenor Square.
About 35,000 journeys were made on the e-scooters between June 7 and July 4. The average journey was 1.8 miles, and the average ride time was 24 minutes – costing about £5.
TfL is trying to use the trial to highlight the fact that privately owned e-scooters remain illegal to use other than on private ground.
A total of 1,103 e-scooters were seized by the Met’s traffic police in June, and a further 320 last month - bringing the total for this year to more than 2,300.
TfL is now considering how the vehicles can be recycled.
Last week the TfL board was told that since July last year, there had been more than 420 offences reported by people riding e-scooters, ranging from robbery, drug offences and youth violence. Private e-scooters have also been caught doing more than 40 mph. Reckless behaviour by e-scooter riders is “commonly reported”.
Last month Mayor Sadiq Khan pleaded with Londoners not to use e-scooters after being challenged over the “Wild West” spread of the vehicles.
Mr Khan said: “E-scooters are against the law. For those who have been given them as a Christmas present or a birthday present, I plead with you not to use them.”
His remarks came after Junior Shay Alexander, 16, was killed by a car driver in a hit-and-run while riding an e-scooter in Bromley, and a three-year-old girl was left with life-changing injuries after being struck by an e-scooter rider in Myatt’s Field Park, Lambeth.
Will Norman, London’s Walking & Cycling Commissioner, on Monday said the TfL trail had “safety at its heart”. The scooters are limited to 12.5mph.
He said: “E-scooters have been on our streets for some time now but with very little regulation. As we look to our capital’s future, we want to ensure a green and sustainable recovery from the pandemic.
“We know that a huge portion of car journeys in London are for very short distances, and we want to explore how e-scooters can act as an innovative alternative.”
Melvyn Caplan, Westminster’s deputy leader, said: “Westminster City Council has worked hard over recent months to identify suitable parking locations for this scheme, and we have monitored the roll-out in other boroughs to ensure that the trial in Westminster can operate as smoothly and safely as possible.”