Transport for London has been warned that proposals put forward for a safety feature to make electric buses more audible risk confusing vulnerable road users.

TfL has commissioned Aecom to come up with a recognisable noise that will help alert pedestrians and cyclists to the presence of vehicles that can be dangerously quiet.

The first electric London buses are due to be fitted with the sound in the autumn and it is hoped other parts of the country will follow.

But the possible options, including a bubbling noise and intermittent bleeps, have been greeted with scepticism by experts and campaigners.

John Welsman, from the policy team at Guide Dogs UK, who attended a TfL presentationlast month, described the sounds as “all very spaceshippy” and said he would prefer electric buses to be fitted with a canned recording of the old Routemaster bus.

Welsman added: “They did play us a sound like someone blowing bubbles through a pipe. That just wouldn’t work. And there was an intermittent bleeping sound like an email alert that would increase or decrease in rapidity depending on the the speed of the vehicle. It was very irritating.”

“As a blind person I could spot the old Routemaster a mile off, because it was so distinctive, but that’s not what they are suggesting.”

He said most of the other six samplers sounded like futuristic vehicles from sci-fi films.

“TfL want to know if they can create a sound that is not necessarily like a combustion engine, but is something new and unique. The feedback from the room was that it would still need to be indicative of speed, direction, acceleration and deceleration of the vehicle.”

Gloria Elliot, chief executive of the Noise Abatement Society, was also played the sounds. She described most of them as “ghastly”, but preferred a sound that most resembled a diesel engine. “Everyone said that’s better, because it wasn’t so irritating, but I don’t think any of them were great.”

She added: “If they had a quieter version of the sound of a Routemaster bus, that might be a good idea.”

Zoe Courtney, policy and campaign officer at the Royal National Institute of Blind People, said she hoped that whatever TfL adopted was also used across the UK.

“The near-silent sound levels on electric and hybrid buses make them a potential danger to blind and partially sighted pedestrians, so it’s imperative that the right sound is added to these vehicles. We did make the point about the importance of having consistency throughout the UK in the meeting, so I’m hoping TfL have heard that.”

Courtney added: “There is a danger that there is going to be so many different wacky sounds from lots of different companies that it going to be really hard to work out what is a vehicle and what is something else in a city.”

Alan Benson, chair of Transport for All, which champions the travel rights of disabled and older people, wanted TfL to show more urgency. “We’ve been asking for a sound on electronic vehicles for five years. I don’t know why it is taking so long. Let’s just get it out there and test it and improve it later if required.”

But Courtney said: “Because the sound needs to do so much work it is imperative that rigorous testing and input is carried out before the final choice is made.”

From 1 July, under an EU regulation, all new models for electric vehicles seeking approval will have to emit a noise, known as an acoustic vehicle alerting system (Avas). Existing electric vehicles will need to be retrofitted with the sound from July 2021.

Tony Bowen, of Brigade Electronics, a firm that spent three years developing a “quiet vehicle sounder”, said permissible frequencies and modulation under the regulation make it clear that the Avas should mimic an internal combustion engine.

Brigade Electronics quiet vehicle sounder designed to alert road users to the presence, speed and direction of electronic vehicles

“TfL keep pursing this aim of something that I believe is outside the legislation. Bubbles, and whistles and bells don’t sound like an combustion engine so definitely won’t be compliant.”

TfL said its bus sound would comply with the regulation but maintained it did not have to mimic an engine noise. A spokesman said: “The regulation references a continuous sound that will increase as the vehicle accelerates, but there is no mention in the regulation that the noise needs to simulate that of an internal combustion engine.”

He said the aim of the project was to ensure all electric buses emit the same sound. “Approximately half of bus passenger journeys take place here and it would make sense for electric buses around the UK to share the same sound.”

However, Elliot urged TfL to adapt Bowen’s vehicle sound for buses. “I don’t know why they are trying to reinvent their own sound at great expense when there’s a product on the market that is perfectly good. They could just tweak it a bit to sound like a bus.”