A door opening on a moving train has caused major disruption on the Piccadilly line .

London Underground said the incident happened on a train approaching Heathrow Terminal 4 on Saturday (January 16) morning, at about 5.30am.

It said a full inspection of the fleet had been carried out, which showed it was an isolated incident.

However, it is understood some drivers were unwilling to return to work until they were satisfied with assurances it was not a widespread problem.

That led to a shortage of drivers on Sunday, which Transport for London (TfL) has blamed for delays and suspensions on the line.

London Underground operations director Pat Hansberry said: "We are very sorry for the delays to Piccadilly line services this weekend. Following an isolated problem with one train door we carried out a full inspection to confirm there wasn't a wider problem, which resulted in some cancellations.

"We are now working hard to restore a full service on the line. We will be paying automatic refunds to our customers who have suffered delays to their journeys."

A TfL spokeswoman said the driver had suspended the train at Heathrow Terminal 4, where all passengers disembarked, after a light in the cab indicated a door was open.

She said the train was mostly empty at the time and no-one had been injured.

The Rail Maritime and Transport (RMT) union has reportedly said this is is the fifth time such an incident has happened in two years, and has called into question the safety of the "ageing fleet" on the Piccadilly line.

TfL has been reporting "severe" delays on the line throughout Sunday.

In an update shortly after 4pm, it tweeted that there was no service between Cockfosters and Uxbridge , and severe delays between Acton Town and all Heathrow terminals, due to a "temporary shortage of train operators".

It said tickets were being accepted on local buses and Great Northern trains.

Anyone planning to use the Piccadilly line is advised to check for updates on Twitter from @piccadillyline.