The operator said where long-distance services were able to operate, they would be “extremely busy” and there was a possibility of short-notice cancellations or alterations.

On Saturday, long-distance services will still run between London Paddington and Reading but at a reduced timetable. There will also be no stopping services between London Paddington and Reading MTR on the Elizabeth line.

GWR operations director Richard Rowland called the operation “really complex”.

“We will run an hourly service from the south-west of England – Cornwall and Devon – and an hourly service from South Wales, which will go into Euston,” he said.

“We will also have trains to Ealing Broadway for those making more local journeys.

“They will be able to change on to the London Underground and into London that way.”

Further work will take place in November and December as part of Network Rail’s performance improvement plan in the Thames Valley.

It includes drainage upgrades for areas of the line that frequently flood, as well as replacing track and sleepers, which support the rails.

Network Rail’s western route director Marcus Jones said the upgrades were “vitally important”.

“These upgrades are essential to our goal of reducing disruption and improving the resilience of this stretch of line into the future.”

Chiltern Railways advised customers to plan ahead before they travel.

“Chiltern services between Oxford and London Marylebone are expected to be busier than usual this Sunday as Chiltern is the only train operator running a direct service between Oxford and London due to engineering works,” a spokesperson said.

GWR has also said there will be disruption on 24 November and 1 December, external.