A group of travellers has moved on to the grounds of £30,000-a-year Marlborough College where the Duchess of Cambridge studied.
A convoy of seven caravans and other assorted vehicles pitched up on the college's sports field on Thursday evening, said police.
College master Jonathan Leigh said: "We are pursuing all legal steps to remove these people as soon as possible."
Kate took three A-levels while at the prestigious college in Wiltshire, which was founded in 1843.
The travellers have told Wiltshire Police they intend to move on "no later than Sunday".
A spokesman said: "We received a call yesterday, shortly before 5pm, from a member of the public saying that up to seven caravans were heading towards the M4. They turned away from the M4 and went to Marlborough.
"We then received a call to say that they had set up on a playing field, which is next to Marlborough College and is privately owned by the college.
"We have been down and spoken to the travellers, who have said it is their intention to leave no later than Sunday, so we are aware of the situation and are monitoring it closely."
As well as the caravans, the travellers have parked a lorry and trucks towing horseboxes on the playing fields.
The independent school offers education for day and boarding pupils up to the age of 18. Kate studied for her A-levels at the college and left in July 2000.