Eton College has been forced to delay the start of the new school term after toilets in its boarding houses backed up due to flooding.
Pupils had to study online due to an overloaded sewer system following heavy rains, Thames Water has said.
Students were due to return to the £46,000-a-year private school on Tuesday and the school has apologised to parents.
An email sent to parents and guardians at the school, which was first reported by Bloomberg, said: “I am very sorry to say that Thames Water have just alerted us to the fact that their sewerage drains are backfilling due to floodwater.
“The sewers in the centre of Eton won’t cope with the arrival of nearly 1,350 boys.”
The email continued: “We cannot safely operate as a school until Thames Water have resolved the issues around our sewerage drains.”
Eton College is one of the UK’s most prestigious schools and its alumni includes many former prime ministers, including Boris Johnson and David Cameron.
Alumni at the school also include Prince William and Prince Harry.
A spokesperson for Eton told the Standard: “Following extensive flooding in the region, the Thames Water sewers which serve the town of Eton flooded.
“Therefore boys could not return for the scheduled start of term on 9 January and the college has moved to remote teaching.
“We are in regular contact with Thames Water as they seek to resolve the situation and we look forward to welcoming boys back as soon as possible.”
Thames Water told the Guardian newspaper its engineers are responding to reported sewerage issues at Eton college.
The company said in a statement: “In this instance, the recent heavy rainfall, along with high groundwater levels and river flooding, caused our local sewer system to overload.
“We are sorry to staff and students who have been impacted. Our teams will be carrying out a clean-up in the coming days once the river levels recede.”
