Prime minister Boris Johnson has caused controversy by suggesting that allowing driverless trains should be a condition of any future funding of Transport for London (TfL).
Mr Johnson made the comments during a site visit to a planned rail manufacturing facility, as part of his 'New Deal' campaign.
He told reporters: 'You can run these trains without the need for somebody to be sitting in the driver's cab the whole time. So what I will be saying to the London transport authority is let's take advantage of this technological leap forward, let's not be the prisoners of the unions any more, let's go to driverless trains and let's make that a condition of the funding settlement for Transport for London this autumn.
'That's the way forward for this country, and we want to make use of the fantastic technology we've got and provide a better service for people in the capital and take the whole economy forward.'
The comments sparked a strong reaction from train drivers' union Aslef, which dismissed Mr Johnston's call as 'nonsense'.
A spokesman said: 'As always, Boris Johnson is talking nonsense about driverless trains. Slashing government funding to TfL means that they cannot afford the signalling upgrade and other technology that would be needed for driverless trains.'
Some trains on the Transport for London network are operated in semi-automatic mode, but are not driverless, as drivers operate doors but are not responsible for acceleration or braking unless problems occur.
TfL's Docklands Light Railway does not have drivers in cabs but uses onboard attendants who control the doors.