Virgin Trains has cut speeds on its pioneering tilting trains because of braking problems - less than three months after its 125mph service began.
Sir Richard Branson at the launch of the new trains
The 110mph limit on Pendolino trains on the London to Glasgow West Coast Main Line was imposed after a warning from the Health and Safety Executive (HSE).
It comes after two trains hit buffers at Liverpool's Lime Street station, slightly injuring some passengers.
Virgin said it was a "precautionary" move while an investigation took place.
The 125mph service was introduced - after several delays - at a ceremony on 20 September attended by Tony Blair.
But on 28 October, a Pendolino was damaged and several passengers were slightly hurt went it hit a buffer at Lime Street station.
FACTS ABOUT TILTING TRAINS
British Rail tried and failed to launch tilting trains in the 1980s
The first Pendolino train was launched in July 2002
The trains were originally forced to run in non-tilt mode
Features on the trains include video and audio entertainment
Virgin Trains claim journey times will soon be cut by 25%
It was followed by second buffer strike there
five days later.
Virgin said drivers had also experienced braking problems at Manchester Piccadilly station and in other places.
And there have been two recent instances of Pendolinos going through red signals in Warwickshire - the first at Rugby on October 21 and the second at Nuneaton on October 25.
The improvement notice to Virgin from the HSE said: "The braking arrangements for your Class 390 trains are not suitable and sufficient in conditions of low adhesion to ensure, so far as is reasonably practicable, the safety of employees, other railway staff and passengers."
On Thursday, a Virgin spokeswoman said: "Following the two incidents where a train has made contact with the buffer stops at Liverpool Lime Street at a very low speed, Virgin confirms that it's carrying out a full investigation with Network Rail and (the manufacturer) Alstom.
"Virgin Trains will also participate in the independent inquiry being undertaken by the Railway Safety and Standards Board.
"As a precautionary measure while the investigations
take place and we understand fully the issues, we have rebriefed drivers on low-adhesion driving techniques (to counter the problem of leaves on the line) and have revised the stopping point for trains at stations.
"We have also temporarily reduced the speed of these trains to 110mph."