The company said it was in the dark as to the extent of the alleged anti-trust violations and did not know what the potential costs might be.
US lawyers have advised the company to have ready significant defences should any proceedings be instigated, but they could not say what the outcome would be.
No formal charges have been made as a result of the investigation, which extends to several US plastics companies, including Comet Products and Comet California Cutlery, both Waddington operations, and to some of their employees.
The businesses, which make plastic cups, plates, cutlery and straws, are part of Waddington's biggest division, responsible for 41 per cent of group sales. The US operations represent a fifth of turnover.
In the six months to October, Waddington announced a 59 per cent increase in profits from plastic packaging after market-share gains and the acquisition of Carthage, a Texas-based plastic cup company.
Until yesterday's announcement, Waddington's shares had been strong performers so far this year following speculation that Ideal Toys, a French-owned toy group, was interested in buying the company's games business.Reuse content