California's attorney general has launched an investigation into allegations of unfair competition practices by the three grocery chains at the centre of a labour dispute in Southern California.

Albertsons, Kroger's Ralphs and Safeway's Vons are accused of entering into a deal to divide strike-related costs and revenues, which may violate antitrust laws, reported Reuters.

"This investigation will seek to determine whether the stores on the other side (of the labour dispute) are playing fair and within the rules set by state and federal law," Attorney General Bill Lockyer was quoted by Reuters as saying.  

The attorney general's office said it had subpoenaed the retailers to produce any revenue sharing agreement after the grocers did not provide it voluntarily.