An £8.6bn upgrade to the main west coast railway line which will entail severe disruption to passengers travelling at weekends over the summer was approved yesterday.

The Office of Rail Regulation said it would be "closely monitoring delivery of the plan" by Network Rail which aims to finish the engineering works by next December.

The revised plan follows widespread disruptions over the festive period caused by overruns which led the regulator to impose a record £14m fine on Network Rail.

It stressed that Network Rail must do all it can to minimise passenger disruption.

Earlier this year, Network Rail announced that it would have to close parts of the line on non-bank holiday weekends as well as bank holidays so that engineering work could take place.

Yesterday, ORR access planning and performance director Michael Lee said: "NR's planned improvements will mean better services for passengers across a wide part of the network from December.

"We have scrutinised this plan, making use of expert independent advice, and considering the views of those who run services, representatives of those who use them, and the government bodies that provide funding.

"It is clear that most people want to see the improvements made this year. Passengers will unfortunately face increased disruption while this work is being done, but will benefit from the new timetables in December 2008.

"We are conscious of the adverse impact on rail freight.

"It is vital that Network Rail delivers this plan. We will closely monitor NR's performance between now and December, ensuring the new infrastructure operates reliably and is maintained safely."

Network Rail chief executive Iain Coucher said: "This news puts us on firm ground to press ahead with our plans to complete the £8.6bn project to transform services on the WCML.

"Our plans deliver a big prize for passengers and freight users alike in the shortest time possible with 30% more long-distance services and significantly improved journey times across the west coast route by the end of the year. We recognise that this massive engineering undertaking will continue to cause disruption to services along the route for the rest of the year, especially at weekends and bank holidays but the significant benefits that will be delivered make it more than worthwhile."

Anthony Smith, chief executive of traveller watchdog Passenger Focus, said: "In an ideal world passengers should not have to put up with the increased disruptions on the network, but we believe this is the best outcome.

"The industry must work to ensure passengers are kept up to speed with disruptions on their route. We would also ask that alternative rail services be provided wherever possible."

Passenger Focus is pushing for train operators to compensate passengers who will have to travel by replacement bus services during disruptions.

Mr Smith said: "It is not right that passengers should be expected to pay the same fare when they are not getting the service they paid for."

EWS, Britain's largest rail freight operator, expressed disappointment. Graham Smith, EWS planning director, said he would have preferred a completion date of May 2009 to avoid disruption to businesses. He said: "You can't put freight on rail replacement buses."