The safety of Cambridge University students is feared to be at risk amid a potential flashpoint between historic boat races and a travellers' fair on the same weekend.
Concerns have intensified after a student rower putting their boat away was hit by a metal nut fired from a catapult by teenagers sitting across the river last summer.
Other incidents in previous years have included youths spitting at boats while leaning over the edge of bridges as well as sandwiches and bananas being used as missiles.
Another clash is feared ahead of the May Bumps rowing event which takes place between June 18 and 21; with the Midsummer Fair on between June 18 and 22.
The fair is a popular traveller gathering held on Midsummer Common, opposite many college boathouses, and is one of England's oldest such events dating back to 1211.
