Almost one in 10 delays and cancellations on Britain's railways are caused by crew shortages, according to figures.
A lack of staff was responsible for 9.42% of the 1.93 million incidents of disruption that occurred between April 1 2013 and December 12 last year, data from the Office of Rail and Road (ORR) shows.
Govia Thameslink - which is responsible for Thameslink, Southern and Gatwick Express services - was the worst affected operator with more than 62,000 incidents leading to 13.6% of its delays and cancellations.
London Midland and First TransPennine Express had the joint next highest rate at 13.5%.
The lowest proportion of delays caused by crew shortages was Virgin Trains East Coast at 2.6%, followed by Virgin Trains West Coast at 3.7%.
Southern has accepted it does not have enough staff to carry out day-to-day operations and training without asking crew to work on rest days.
The operator issued a statement in December which read: "We have under way the biggest driver recruitment programme in the UK, alongside ongoing conductor training. This will result in a more consistent service for our passengers."
It added: "There continues to be a risk of cancellations, in particular during periods of high annual leave - but we are steadily overcoming this. In the meantime we will continue to keep you informed of any possible cancellations via our station screens, app and website."
Southern said a shortage of drivers and conductors can also be caused by a number of reasons such as sickness or crews being displaced by earlier disruption.
The process of becoming a train driver takes more than a year. It includes a minimum of 240 hours in practical training and an additional 840 hours learning theory.
In September ScotRail revealed that it had received more than 12,000 applicants for 100 train driver jobs in a week.
The public performance measure used by the rail industry to calculate the proportion of trains that are on time only records long-distance services as being delayed if they arrive at their terminating station at least 10 minutes behind schedule, while commuter services must be at least five minutes late.
Under this measurement, punctuality has improved since 2002 but dipped slightly in recent years, reaching 89.2% for the last 12 months, according to Network Rail.
The Department for Transport said it closely monitors the number of trains that are cancelled or delayed due to a lack of crew and pledged to hold companies to account if "too many" are disrupted in this way.
A spokesman for the Rail Delivery Group, representing train operators and Network Rail, said: "Occasionally train operators are unable to run a journey because a member of staff isn't available.
"Disruption to services, for whatever reason, means that train crews can be displaced and physically in the wrong place to crew a scheduled train. Other reasons, including sickness and annual leave, also have an impact."