Almost one in 10 delays and cancellations on Britain's railways are caused by crew shortages, according to new figures.
A lack of train 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 - is the worst affected operator with more than 62,000 incidents leading to 13.6% of all 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%.
A lack of drivers and conductors can be caused by a number of reasons, such as sickness, annual leave or crews being displaced by earlier disruption.
But Southern has accepted that it simply does not have enough people to carry out day-to-day operations and training without asking crew to work on their rest days.
The operator issued a statement in December which read: "We have under way the biggest driver recruitment programme in the UK, alongside on-going 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."
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.