Services on the two lines privatised last month have deteriorated since the private operators took over, according to figures prepared for the Passenger's Charter.
The number of trains arriving five or more minutes late has increased on services operated by both South West Trains and Great Western Trains.
The worst performance was on the main-line services operated by South West Trains, now part of the Stagecoach empire, where 17 per cent of trains, nearly one in six, were five or more minutes late. During the previous month, the last of BR's ownership, only 10.3 per cent of trains were late and on average, over the past year, the figure was 14.1 per cent.
For South West's suburban trains, the number of late trains doubled as, during Stagecoach's first four weeks, 11.8 per cent of trains were late compared with only 5.9 per cent during BR's last month. The punctuality performance falls well short of its set targets which strive for a level of no more than 10 per cent late trains on its suburban service, and 13 per cent on its main-line services.
Stagecoach's managers will be particularly worried that their trains' performance on main-line services has slipped below the targets at which regular travellers become eligible for refunds. Refunds become payable if the number of late trains is 3 percentage points or more above the target.
A spokeswoman for South West Trains accepted it had been a bad month, but blamed the weather. She said : "It has been a particularly cold month, with some snow, and that has affected the service. We are working hard to improve the service."
A spokesman for Great Western said that a number of factors had contributed to the reduced performance. He said: "There was bad weather, but also we've been unlucky with vandalism and trains have even been delayed by children playing on the line."Reuse content