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Hundreds of trains on the Piccadilly line, including those on the Night Tube, are being delayed from entering service because no driver is available, it was claimed today.

When a train is left without a driver it frequently takes two or more hours to find a replacement and get it back into passenger service.

Staff claim the issue has a serious effect on Night Tube services.

The Evening Standard has seen London Underground documents covering the past three months, which detail the number of train delays with the stated cause “ONA” — operator not available.

The documents, which list more than 200 trains delayed from entering service, cover only a sample of dates, and the overall number of delayed trains is believed to be far higher when every day and Night Tube shift is counted.

London's Night Tube - In pictures 26 show all London's Night Tube - In pictures 1/26 Tube passengers eating fast food products whilst travelling on the night tube service of Victoria line in London Rex Features 2/26 London Mayor Sadiq Khan poses in front of the open doors of a train car during the first Night Tube train to leave from Brixton Underground Station Getty Images 3/26 In full: the Night Tube map TfL 4/26 Tube passengers take a selfie next to a London underground roundel designed for the launching of the Night Tubes on display at Oxford Circus underground station EPA 5/26 Passengers sit on the ground on board the Night Tube along the Victoria line Getty Images 6/26 Passengers pose for a photograph as they wait for the Night Tube train service at Oxford Circus Reuters 7/26 Members of the public wave and cheer as they see London Mayor Sadiq Khan in a tube train carriage in Oxford Circus Underground Station Getty Images 8/26 Camden Town TfL 9/26 British Transport Police pose for photographs on the platform at Oxford Circus underground station with Chris Horton, British Transport Police Superintendent (second from left) Lucy Young 10/26 Passengers cram on to a train through Tottenham Court Road PA 11/26 Camden Town TfL 12/26 Passengers wait to get on a Night Tube train service at Oxford Circus Reuters 13/26 Passengers sit on board the Night Tube along the Central line Getty Images 14/26 Leicester Square TfL 15/26 Network Operations Controller Cem Toygar speaks through his headset at the London Underground Control Centre Getty Images 16/26 Cups are left by a teapot on a tea trolley at the London Underground Control Centre Getty Images 17/26 Balham TfL 18/26 Mayor of London Sadiq Khan travelling with passengers on a northbound Victoria line tube train during the launch of London's Night Tube Reuters 19/26 Evening Standard readers eagerly anticipate the start of the Night Tube ahead of it's launch here at Oxford Circus as they head home Nigel Howard 20/26 Train operator Sharif, 23, takes part in Night Tube trials Lucy Young 21/26 Passengers disembark a Night Tube at Leyton Underground Station Getty Images 22/26 Night Tube: Even British Transport Police officers were thrilled about riding the service all-night for the first time Twitter/BTP 23/26 A neon sign which says 'Night Tube' is on display in Brixton Underground Station Getty Images 24/26 Passengers yawn and sleep as they sit on board the Night Tube along the Victoria line Getty Images 25/26 Workers use computers in the London Underground Control Centre Getty Images 26/26 Night tube: Sadiq Khan visits Oxford Circus and poses up with the new Night Tube logo and an Evening Standard on the Central Line Alex Lentati 1/26 Tube passengers eating fast food products whilst travelling on the night tube service of Victoria line in London Rex Features 2/26 London Mayor Sadiq Khan poses in front of the open doors of a train car during the first Night Tube train to leave from Brixton Underground Station Getty Images 3/26 In full: the Night Tube map TfL 4/26 Tube passengers take a selfie next to a London underground roundel designed for the launching of the Night Tubes on display at Oxford Circus underground station EPA 5/26 Passengers sit on the ground on board the Night Tube along the Victoria line Getty Images 6/26 Passengers pose for a photograph as they wait for the Night Tube train service at Oxford Circus Reuters 7/26 Members of the public wave and cheer as they see London Mayor Sadiq Khan in a tube train carriage in Oxford Circus Underground Station Getty Images 8/26 Camden Town TfL 9/26 British Transport Police pose for photographs on the platform at Oxford Circus underground station with Chris Horton, British Transport Police Superintendent (second from left) Lucy Young 10/26 Passengers cram on to a train through Tottenham Court Road PA 11/26 Camden Town TfL 12/26 Passengers wait to get on a Night Tube train service at Oxford Circus Reuters 13/26 Passengers sit on board the Night Tube along the Central line Getty Images 14/26 Leicester Square TfL 15/26 Network Operations Controller Cem Toygar speaks through his headset at the London Underground Control Centre Getty Images 16/26 Cups are left by a teapot on a tea trolley at the London Underground Control Centre Getty Images 17/26 Balham TfL 18/26 Mayor of London Sadiq Khan travelling with passengers on a northbound Victoria line tube train during the launch of London's Night Tube Reuters 19/26 Evening Standard readers eagerly anticipate the start of the Night Tube ahead of it's launch here at Oxford Circus as they head home Nigel Howard 20/26 Train operator Sharif, 23, takes part in Night Tube trials Lucy Young 21/26 Passengers disembark a Night Tube at Leyton Underground Station Getty Images 22/26 Night Tube: Even British Transport Police officers were thrilled about riding the service all-night for the first time Twitter/BTP 23/26 A neon sign which says 'Night Tube' is on display in Brixton Underground Station Getty Images 24/26 Passengers yawn and sleep as they sit on board the Night Tube along the Victoria line Getty Images 25/26 Workers use computers in the London Underground Control Centre Getty Images 26/26 Night tube: Sadiq Khan visits Oxford Circus and poses up with the new Night Tube logo and an Evening Standard on the Central Line Alex Lentati

“We are talking many hundreds of cases of trains delayed over the last few months alone because there is no driver available,” said a staff member.

The issue is caused when drivers fail to turn up for work because of illness — though unions claim there are not enough drivers to operate the services.

The documents relate only to the Piccadilly line but sources say other night Tube lines, including the Central, are also affected.

During the day, with trains running every two or three minutes, passengers will not notice much difference in service. But on the Night Tube, with fewer trains running, a missing service can result in much longer waiting times.

The staff member claimed: “Night Tube is what really worries us. Staff are confronted with irate, often drunk members of the public late at night waiting for a train with big gaps in service.”

Manuel Cortes of the TSSA union, whose members include ticket staff, white collar and managerial grades, told the Standard: “The situation you have described to me regarding cancellations is something that our members on the ground have been reporting to me. The Underground needs to get its act together as Londoners deserve better than this.”

Nick Dent, director of line operations for the Tube, said: “The Night Tube has substantially cut journey times for customers and has boosted London’s night-time economy. The vast majority of customers continue to experience a good and reliable service, and it is not true that customers have experienced delays of over an hour due to non-availability of train drivers.”

He added: “Where there are any cancellations the timings of trains are managed to reduce gaps between trains as much as possible.”

A Tube spokesman said although certain trains are delayed by there being no driver available, other trains continue to enter the service.