An old engine came to the rescue of 450 passengers who were stranded for more than five hours on one of Greater Anglia's plush new trains, it can be revealed.

On Tuesday, the 7.40am Greater Anglia service from Norwich to London broke down at Forest Gate, just outside the capital, at 9.25am, around the same time it had been due to arrive at Liverpool Street Station.

But more than five hours later, the 450 passengers were still stuck on board after attempts to transfer them to another train failed.

Passengers eventually left the train at around 2.30pm by ladder and boarded another train which took them onto Liverpool Street Station.

Now, it has emerged the train which came to the stranded passengers' aid was a Class 321, one of Greater Anglia's old stock, which the new trains were supposed to replace.

Passengers who wished to return to Norwich immediately were then offered a seat on another old train, a Class 90.

The majority of Class 321 and Class 90 trains were designed and manufactured between the late 1980s and early 1990s.

A spokesperson for Greater Anglia apologised to passengers who were "severely delayed" for the length of time they were caught up and the resulting "major inconvenience".

They said: "We will be holding a full investigation into all aspects of this incident to try to prevent any similar such problems in future.

"The broken-down train is one of our new intercity trains.

"We are working closely with Stadler, the manufacturer of the train, to improve the reliability of our new trains.

"They have dispatched a specialist team of engineers from Switzerland to assist us with this issue."

The problem with the original train is understood to have been a power failure.

It comes amid a string of issues with the operator's fleet of new trains, which have led to delays and cancellations across the county.

On Wednesday, Greater Anglia also confirmed that some of its new Stadler 755 trains were only running on diesel rather than electric, while tests are carried out into the performance of their three-carriage trains.

The bi-mode trains are designed to switch between diesel and electric.