The cutbacks, which yesterday prompted Labour's transport spokeswoman, Glenda Jackson MP, to demand a Government statement, are a serious political embarrassment for the Conservatives in the run-up to the general election.
Delays and cancellations are expected in other regions, as the newly- privatised firms drive through "productivity" deals designed to slash staff levels.
Ms Jackson said: "Rail privatisation is unravelling before the Government's very eyes. With less than six weeks before an election is called, the nightmare of higher fares, soaring complaints and plummeting services has become a reality.
"These cuts prove that ministerial pledges on minimum service levels were not worth the paper they were written on."
South-West Trains, operated by the bus company Stagecoach since January 1996, promised to run 99 per cent of the trains hitherto operated by British Rail.
But since concluding an agreement last month with the footplate union Aslef, under which 70 train drivers - one in 10 - took voluntary redundancy, the company has not had enough crews to meet operating requirements.
Commuters faced delays and cuts last week, and from tomorrow, 39 trains will be cancelled every day for at least four weeks.
The company claimed yesterday that "organised disruption" would be easier for passengers to handle than continuing last-minute cancellations. "We are cancelling trains on less used and less busy routes to make sure we cover the more important routes. The timetable should be operating as normal by mid-March."
The Office of Passenger Rail Franchising tried to play down the disruption, but confirmed that South-West Trains would be fined. "This is an isolated incident. The fact they will be penalised by quite significant sums of money has already focused their minds on putting the problem right."
The scale of the penalty will not be known until after the end of the financial year in April. Some rail industry observers believe the fines could be more than pounds 1m.
Keith Bill, secretary of pressure group Save Our Railways, predicted: "This is only the tip of the iceberg. Stagecoach isn't the only company which is going to make these cuts."Reuse content