Alton Towers has alerted coach companies to let them know it will be closed for a lavish party organised by BT for its employees.
More than £1m will be spent on a party for 25,000 employees and guests at the Staffordshire theme park in June.
The park will be closed for the day and a music concert will be held on the evening.
It has been organised to celebrate the contribution of staff who joined more than 30 years ago when what is now BT was a department of the Post Office.
A spokeswoman from Alton Towers said they had written to all of the coach companies on their mailing list to let them know the park would be closed.
"We haven't been given an exact figure as to how many people will be attending on the day so we don't know for definite whether we'll have to close the park yet," she said.
A BT spokesman said: "We haven't put a final price on the party yet because we don't know how many will be coming.
"It is for people with 30 years service and they can bring their families.
"It is a thank you for their commitment to BT over the years."