High-speed trains taken out of service on two of the UK’s busiest intercity lines have been cleared to return to the tracks, although disruption is set to drag on for “some time”.

More than 150 trains built and maintained by Japan’s Hitachi were pulled from service by Great Western Railway and London North Eastern Railway without warning at the weekend after engineers found cracks on parts of their chassis, causing severe disruption to services.

Ministers, the rail industry and safety experts on Thursday said that many of the Class 800 trains could be safely returned to carrying passengers while a long-term fix is worked on.

Trains with small cracks on lifting points used to raise trains for inspection have been declared fit to run, while some with bigger cracks will be kept back to be repaired. All trains will be repaired in the long run.

Chris Heaton-Harris, the rail minister, said that “many trains” can safely return to the network, but warned passengers to expect disruption to services “for some time to come”.

“Safety is our absolute focus, which is why Hitachi will carry out a comprehensive daily testing regime on affected trains,” he said.

The Class 800 trains, barely five years old and capable of speeds of 140mph, have become the backbone of busy routes linking London and the west of England and Wales, and London and the north of England and Scotland.

GWR, which has suffered the worst disruption, was on Thursday still advising passengers not to travel. The operator is preparing for 60 of its 93 Class 800s to return next week, although there will be some cancellations and timetable changes.

LNER, which has also been hit, was running a reduced timetable on Thursday, and said its Class 800s would begin to return next week. Hitachi-built Class 385 trains, which run in Scotland, were on Thursday back in service following inspections.

“The next step on the route back to normal service levels will be for Hitachi to present their long-term repair plan for the fleet. We expect to see this shortly,” Heaton-Harris said.

The chaos has been a severe embarrassment to the Japanese conglomerate, which manufactured the class 800 trains in County Durham as part of a push into the UK rail industry.

Hitachi and the train operating companies have refused to disclose to passengers how many of the 182 Class 800s have suffered some form of cracking, or how many are still considered too dangerous to return to normal use.

But a person close to the industry discussions this week said only a “very small portion” will not be able to return to carrying passengers.

Engineers were not concerned about the overall structural integrity of the trains, but rather that some parts could fly off during journeys, injuring passers-by or damaging track.

“Safety remains our number one priority, and we and our partners have worked round the clock to agree an approach that allows the return of trains to service where they have been deemed safe,” said Andrew Barr, Hitachi Rail’s chief executive.

Anthony Smith, chief executive of passenger group Transport Focus, said the industry should release as much information as it can to help restore passenger confidence.

“When you have a crisis of confidence like this, it is best to be as open as possible,” he said.