Over-running engineering work caused major disruption to rail services on some of the busiest routes in the country on Monday.

The problem, in a tunnel near West Hampstead in north-west London, hit Thameslink journeys between Bedford and London St Pancras International.

Two of the four lines were closed, resulting in trains being cancelled or delayed, with disruption expected to continue until noon.

East Midlands Trains from London St Pancras were also affected.

The disruption came on the first day of timetable changes to cater for autumn leaves.

Thameslink's owner, Govia Thameslink Railway (GTR), is embroiled in a bitter dispute with the Rail, Maritime and Transport (RMT) union over the role of conductors at its Southern franchise.

RMT leader Mick Cash said of Monday's disruption: "If GTR'S parent company weren't hoarding £100 million in profits, that money could be invested in more engineers and more regular maintenance, which would help us avoid these regular, catastrophic infrastructure failures."

Mr Cash was due to give evidence on rail franchising to the Commons Transport Select Committee later on Monday.

Passengers at West Sutton were among those affected but after getting on a late-running service they suffered further delays when a vehicle hit a bridge.

The incident affected trains to London Blackfriars.

A Network Rail spokesman said: "We are sorry for the major disruption caused today to journeys between Bedford and London St Pancras International, which is due to our over- running engineering works in a tunnel near West Hampstead.

"We are doing all we can to reduce the disruption this causes, complete the work and return services to normal as quickly and safely as possible."

A rail replacement bus service was in operation between St Albans and Hatfield, and Thameslink tickets were being accepted by other operators.