Detectives were called to Stowe School, whose former pupils include billionaire Virgin Group founder Richard Branson, after other members of staff raised the alarm about strange smells in the food last Friday.
"During routine checks, school staff became suspicious that foodstuffs had been tampered with and the police were alerted," said a spokesman for Stowe, set in 750 acres of impressive grounds near Buckingham, 60 miles (100 km) northwest of London.
"We continue to cooperate closely with the police in their investigation." A Thames Valley Police spokeswoman said: "It was soup. It was being prepared for the pupils." Detectives said a 58-year-old man had been arrested at his home on suspicion of administering poison with intent to endanger life or inflict grievous bodily harm.
He has not yet been questioned because of ill health and has been bailed until March 29.
Media reports said the man was a kitchen porter and police confirmed he was a member of staff at the school, which has about 750 boys and girls, aged from 13 to 18, and where fees are more than 27,000 pounds ($41,500) a year for boarders.
"Nobody has been harmed, but these are serious accusations," said Detective Constable Vicky Ball. "Our investigation is still in its early stages and forensic work is continuing."
