The news has been made public by P&G itself. The firm alerted Unilever after a corporate intelligence firm it had hired overstepped internal guidelines.
The two companies are old sparring partners and have clashed many times in the past, most notably in what have been dubbed "the soap wars". Both companies produce a number of competing brands in the soap powder market.
Lawyers for the two firms are now negotiating some kind of settlement. According to one report, Unilever could be seeking tens of millions of dollars in compensation.
The intelligence firm was briefed to collect information on Unilever's US haircare business. It is understood, according to reports, that the firm employed tactics which included going through Unilever's rubbish looking for information.
P&G produces the Pantene range of haircare products, while Unilever is behind such brands as Organics and SunSilk.
In a statement, P&G said, "The specific means used to obtain the information [on Unilever] violated the company's strict policy regarding competitive business information gathering."