Rail passengers have been warned to "only travel if necessary" next week after a national emergency was declared by the UK Health and Security Agency.

Greater Anglia has issued the guidance for Monday, July 18 and Tuesday, July 19, after a red weather warning was announced as temperatures could reach up to 40C in some parts of Norfolk.

A red weather warning is only put in place when a heatwave is so severe and prolonged that it could impact the UK's health and social care system.

At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups.

Network Rail is imposing speed restrictions across the entire Greater Anglia network to ensure the safe running of trains and protect the rail infrastructure from damage.

Buckled rails, sagging overhead lines or other major faults as a result of the temperatures could block lines and cause heavy delays due to high temperatures.

Many journeys are expected to take longer, as trains will be travelling at slower speeds than usual, and service alterations will take place on many routes.

Passengers are warned to expect widespread disruption and short-notice cancellations, especially from the middle of the day onwards when the hottest temperatures happen.

A reduced service will operate between Norwich and London Liverpool Street.

A normal timetable is currently planned for regional services between Norwich and Sheringham, Great Yarmouth, Lowestoft and Cambridge/Stansted Airport.

Passengers who do choose to travel on Monday and Tuesday have been urged to wear cool clothing, have plenty of water and consider using hand-held fans. They should allow plenty of time for their journeys and expect cancellations and delays.

Water fountains with free cold water are at 13 of Greater Anglia’s stations and at London Liverpool Street.

Jamie Burles, Greater Anglia's managing director, said: “We are very sorry to be disrupting our customers’ travel plans, but the safety of our customers and rail staff is of paramount importance.

“Our trains will be running at lower speeds because we want to minimise the risk of heat-related damage to the railway tracks and signals, which could then cause huge delays in record-breaking, sweltering temperatures putting our passengers at risk of falling ill due to the heat.

“While we have air conditioning on the majority of our trains now, if they are held up in long delays and the power to the train is cut off, temperatures would rise and it could become very uncomfortable.

“Please listen to the weather warnings, stay cool, drink plenty of water and avoid travelling on Monday and Tuesday.”