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A couple have been arrested after being caught having sex on the Queen's lawn.

Tourists visiting Windsor Castle were shocked when the couple, in their 30s, stripped off near the castle's Garter Tower and started having sex, the Guardian reports.

Witnesses said the couple were in full view of the pub and shops, and didn't stop what they were doing despite onlookers shouting at them.

"It was going on for about 10 or 15 minutes, which is quite a long time considering the location," witness Mark Robinson told the newspaper.

He said it was only when police arrived that the man stood up, pulled his trousers back up and helped the woman with hers.

Afterwards, Japanese tourists were comparing videos of the incident, he said.

A Thames Valley police officer confirmed that the pair were arrested and cautioned for 'outraging public decency'.

- NZ HERALD STAFF