If you are short you are reliant on your drivers working overtime - there is no other way around it

Spokesman, Thames Trains

However, the overtime is not being worked because drivers "wanted to spend time with their families".

Buses have replaced services on three of the company's branch lines.

A spokesman for Thames said that 52 new drivers were currently in training, but in the short-term there was no other way to cover the shortages.

"If you are short you are reliant on your drivers working overtime - there is no other way around it," he said.

"If you talk to any other business in London and the south east then they will tell you they are short of staff.

"These staff shortages have a far greater impact on our customers than in, say, a supermarket, where people just have to wait longer in a queue."

'Stressed drivers'

He also said stress may have played a part in the decision of many drivers to work their normal hours this week.

A Thames Trains driver was one of the 31 people who died in the Paddington rail crash in October 1999 after he missed a red signal.

The spokesman added: "It is fair to say that train drivers have been under stress in the last year so it is not surprising that when half-term comes along they want to spend time with their families."

The company was doing all it could, he said, to address the problem of driver shortages.

"We are 31 drivers short and 10 are on long-term sick leave but we have recruited 78 drivers in the last 18 months," he said.

Worst records

"The problem is that it takes 11 months for a driver to qualify."

The company had one of the worst records for punctuality in London and the south east during the summer, with only 73.8% of trains arriving on time between June 24 and July 21.

The spokesman said this week's cancellations were mainly affecting off-peak services.

Replacement buses are operating during the day between Marlow and Maidenhead, Reading and Basingstoke, and Reading and Newbury.