A new inquiry into the safety of trains on London Underground's Central Line has been ordered by mayor Ken Livingstone.
Defective bracket found after testing
Unions raised concerns after cracks were found on brackets, fitted to solve a problem which led to an accident in which 32 people were injured.
The line was closed for over three months after a motor fell off, causing a train to derail at Chancery Lane station in January.
New safety brackets were fitted to the trains to keep motors securely in place in the wake of the accident.
But safety fears were raised last month by the Rail Maritime and
Transport (RMT) union and engineers have since been checking for cracks on the new brackets.
Senior London RMT official Bobby Law said they wanted all trains to be shut down and checked by indepedent inspectors or a 40mph limit imposed.
He said: "We don't believe it (the Central Line) is safe, our train drivers don't believe it is safe and our safety representatives don't believe it is safe."
Now Mr Livingstone has ordered an investigation into the general state of the 10-year-old Central Line fleet.
He said an "intensive safety regime" was necessary and the trains would require "detailed watching" for a long time.
Cracks 'not unnatural'
London Underground (LU) has maintained that trains on the Tube are all safe to operate.
Spokesman Bob Bayman told BBC London a very small number of cracks had been found, which was "not unnatural" in this type of equipment.
"We would withdraw the trains if we believed they were unsafe, but the fact is these trains are safe to run," he said.
"There are concerns over their reliability, these are by far from the most reliable trains we have ever had, but they are safe.
"Our job is to keep London moving and these trains are fit for that purpose."