Rail bosses have been told to plan for up to 10 days of strikes over plans to introduce driver-only operation.
The report, by the Rail Safety Standards Board, has warned operators to prepare for five to 10 days of action over plans to remove rail guards from trains.
The report states safety factors are “not quantified” and that introducing driver only operation could mean that there was a “potential for increased fare evasion.”
The RMT union is at loggerheads with First Great Western over the plans which will affect people travelling down to the South West.
RMT general secretary Mick Cash said: “This report shows that the train companies are preparing to cause chaos and shut down railways for days on end in a profit-led gamble on Driver Only Operation.”
A spokesman for the Rail Delivery Group, which represents train operators and Network Rail, said: “The rail industry regularly explores ways to get better value for passengers by running services more efficiently, but safety is always the top priority which is why ours is now the safest railway in Europe for passengers.”