Rail passengers faced weeks of chaos because the rail industry was not properly prepared for timetable changes, a government watchdog says.

Trains were cancelled and delayed across the network after new timetables were brought in on May 20.

Among those affected were people using Great Northern services on the line between King's Lynn and London.

Govia Thameslink - which runs the Great Northern franchise - amitted customers faced 'unacceptable levels of disruption'.

Prime Minister Theresa May said the disruption was 'totally unacceptable'.

Transport watchdog the Office of Rail and Road (ORR) was asked to investigate.

Its findings, out today, say: 'Neither GTR nor Northern were properly aware of or prepared for the problems in delivering the timetable and they did not do enough to provide accurate information to passengers when disruption occurred.

'The inquiry has determined that during the planning stages the industry placed engineering and planning concerns ahead of serving its passengers, and that was made worse by the poor information train operators provided when disruption happened.

'The DfT's decision to agree to phase the introduction of Thameslink stretched resources at Network Rail's timetabling department and that the industry, as a whole, failed to foresee that these combined factors created a serious risk that the revised timetable could fail.'

ORR and inquiry chairman, Prof Stephen Glaister said: 'The May 2018 timetable was meant to offer more services and reliability, but in reality it led to major disruption for passengers. Today's report uncovers the issues that Network Rail, GTR, Northern, ORR and the DfT together need to address to stop this disruption happening again.'

GTR chief executive Patrick Verwer said: 'We welcome the preliminary investigation into industry-wide issues surrounding the introduction of the May timetable and will look at the findings to consider how we can make improvements for passengers.

'We are very sorry for the disruption the new timetable caused and have already contacted over 50,000 people offering up to a month's travel cost in compensation over and above the regular money they can also claim.

'Thameslink and Great Northern services are now back on track with over 87pc of Thameslink and Great Northern services on time last week. The average for the last four weeks was 84.5pc, which is similar to the period before the timetable change.'

Here is a timeline of this summer's disruption:

May 20: Govia Thameslink Railway introduces timetable changes which will add an extra 400 trains to the network. It pledges 'passengers will see huge benefits'.

May 24: GTR announces an interim timetable, saying: 'While nationwide, more than eight out of 10 services have arrived as planned since the new timetable was introduced, customers in some parts of the country have experienced unacceptable levels of disruption.'

June 3: Network Rail, Northern and GTR say they are 'urgently working on comprehensive plans to reduce disruption'.

June 15: GTR chief executive Charles Horton resigns. He says: 'I recognise that passengers have been hugely frustrated at the significant disruption caused by the introduction of new timetables.'

July 6: GTR publishes a further amended timetable.

August 28: GTR extends compensation scheme to all passengers.