Passengers on Southern, Thameslink and Great Northern services have suffered the worst rail disruption in the UK, according to a highly critical report.
The National Audit Office (NAO) said the UK's largest rail franchise had not provided value for money.
It found the services had been the "worst on the network" since Govia Thameslink (GTR) took over the routes.
The Department for Transport (DfT) said industrial action had caused a shortage of train crews.
It is being blamed for not ensuring adequate staffing before awarding the franchise.
Government spending watchdog the NAO said GTR had too few drivers when it was awarded the Thameslink, Southern and Great Northern franchise.
It added that since GTR started operating the full franchise in July 2015, 7.7% of services – about 146,000 services – had either been cancelled or delayed by more than 30 minutes, compared with 2.8% on the rest of the network.
Of these, more than one third were caused by train crew shortages.
Amyas Morse, head of the NAO, said: "Over the last three years long-suffering passengers on the Thameslink franchise have experienced the worst performance on the rail network.
"Some of the problems could have been avoided if the Department [for Transport] had taken more care to consider passengers in its design of the franchise."
A NAO spokesman said the DfT "did not seek sufficient assurance that Govia Thameslink would have enough train drivers when it took on the franchise".
"Govia Thameslink received fewer drivers than it expected from the previous operator of the Thameslink and Great Northern routes, and driver shortages have persisted on Great Northern services," he added.
The DfT admitted disruption to passengers was "unacceptable", but said services had improved over the last 12 months.
A spokesman said: "The primary cause of delays and cancellations to passengers has been lack of available staff, which is a direct result of strike action."
GTR said Southern, Thameslink and Great Northern was one of the UK's largest rail franchises, carrying almost one million passengers a day.
Charles Horton, chief executive of GTR, said: "The report identifies numerous root causes for the challenges it has faced.
"These difficulties have sometimes been greater than expected and we regret the disruption caused to our passengers."
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