The London Ambulance Service computer system crashed for nearly two hours on one of the busiest nights of the year so far.

The crash hit the LAS on Saturday night, when a fault occurred in the computer-aided dispatch system. This feeds emergency calls into a computer and automatically alerts the nearest ambulance.

As technicians worked to fix the fault, incoming emergency calls had to be handled manually, with radios used to communicate with paramedics located by maps.

The crash happened at 22:40 and was resolved almost two hours later at 00:30. But the LAS has denied newspaper claims that as many as 25 patients were left waiting up for to an hour for medical help.

A statement issued to E-Health Insider said: “The Service experienced technical problems with its computer system for just under two hours.

“We reverted to our tried and tested process of recording the details of emergency calls on paper and passing the information to ambulance crews by radio. We were able to maintain normal operational service and provide excellent patient care throughout this period.

“No calls were lost or stuck in the system and we are not aware of any delays caused by the computer problems.”

However, shadow health secretary Andrew Lansley has called for an urgent investigation into the failure.

He said: “After past problems with the introduction of computerised call handling this is a disturbing evidence of problems in the system. We will be looking for the team at the head of the London Ambulance Service to account for these events.”

The CAD system was at the centre of controversy when it was implemented in 1992, since it led to delays of up to 11 hours that reportedly claimed up to 30 lives.

Since then, it has suffered periodic problems. In 2006, the system crashed nine times in two weeks due to a software upgrade. A further crash, blamed on a hardware fault, was reported in August.

The LAS says that an internal investigation looking into the most recent crash has been launched.

Related: London ambulance computer crashes nine times.

Link: London Ambulance Service