Tram services in the West Midlands have been suspended indefinitely after a fault was discovered among the fleet.
West Midlands Metro, which runs the network between Birmingham and Wolverhampton, said all of its 21 trams had been removed for inspection.
Alternative travel arrangements have been put in place.
As a result of the problems, the Conservative group on Birmingham City Council has called for a delay to the city's Clean Air Zone charges.
Payments, which will affect people who drive high polluting vehicles into and out of the city centre, are due to begin on 14 June and the aim is to cut pollution.
Robert Alden, leader of the Conservative group, said introducing charges when one of the "key pieces of public transport" was out of action would hurt businesses and people who would otherwise have to pay to use their car.
The tram operator said the decision to stop services was precautionary and "had not been taken lightly" and passengers would be helped to find local bus and rail services instead.
Eight million journeys were made on the network in 2019-20, according to figures from the Department for Transport.
West Midlands Metro apologised for the inconvenience and said inspections were already under way to resume services as soon as possible.
Metro tickets will be accepted on the following services:
- The number 74 and 79 National Express buses between Wolverhampton, West Bromwich and Birmingham
- West Midlands Railway and Avanti rail services between Wolverhampton and Birmingham
The tram operator was informed of the fault following maintenance checks from the manufacturer, CAF.
Midland Metro Ltd has not revealed the nature of the fault, nor the number of trams affected, but said staff and customer safety had never been compromised.
Passengers can keep informed of the latest developments on the West Midlands Metro website.
The tram service normally operates every 6-8 minutes during peak times between Birmingham Library and Wolverhampton St Georges, travelling through Handsworth, West Bromwich and Wednesbury.
The Midland Metro started operating in May 1999, with 4.8 million journeys in its first year of operation.
The network is currently in the middle of a £1.3bn expansion programme, led by the West Midlands Combined Authority and Transport for West Midlands.
Under their plans, West Midlands Metro is expected to carry passengers to Dudley and Brierley Hill by 2023.