Prince Harry can count himself lucky to be back at Eton this week - under the school’s drugs policy, he could have been kicked out.

In a statement, Eton head John Lewis said that he would expect parents to deal with drug use in the holidays.

But a copy of the drugs policy seen by The TES says: “The headmaster has the power... to dismiss any boy for possessing, using, supplying or being otherwise involved with any illicit drug or other harmful substance at any time during his school career.”

It continues: “The headmaster has the power to treat as a breach of school discipline behaviour during the leaves and holidays of a boy’s school career which is liable to bring the school into disrepute.”

However, sources close to the school said that these powers were only held in reserve and not generally used.

Eton has a reputation for a tough stance against drug use, and pupils caught red-handed in term-time still face immediate expulsion.

Other private schools have similar policies. Edward Gould, the master of Marlborough College, where pupils are given urine tests, said: “I would view each case on its own merit, though any pupil bringing drugs into the school would leave,” he said.

A spokesman for Rugby School said: “If any child was caught with drugs on them they would get expelled. If it was shown that they had been using drugs without actually being caught red-handed, they would be subjected to periodic urine tests.”

The Independent Schools Council said that private schools in general were now taking a more flexible stance.

Dick Davison, joint national director of the ISC’s information service, said: “An increasing number will take a fairly pragmatic view. Many will say that if it’s clearly just a teenage experiment that is unlikely to be repeated, they may well decide that, subject to a disciplinary regime, they can stay in school.”

But he stressed that supplying or dealing in drugs would lead to expulsion.