Greene King Pub Company is facing the threat of legal action after tests confirmed that Aftershock bottles at one of its pubs contained antifreeze, detergent and a sugar solution.
The Bear and Ragged Staff, in Crayford, Kent, hit the headlines last month after two customers and two staff were taken violently ill after drinking shots from a back-bar display bottle of Aftershock.
Tests by Bexley Council officers revealed the bottle contained antifreeze, which can cause kidney failure, blindness and death.
Greene King has now e-mailed all managers in its 520-strong managed estate to brief them on the situation and is carrying out interviews with staff at the pub.
Other display bottles of Aftershock at the pub contained what is believed to be washing-up liquid and others a "sugar-based solution with a small amount of alcohol".
A spokeswoman for Bexley Council said: "An urgent investigation is currently ongoing to identify those responsible.
This may ultimately result in legal proceedings, because of the serious nature of the incident."
Greene King is facing the possibility of legal action from both the council and the two customers even though it believes it does everything possible in its health and safety training to stop the situation arising.
Mike Gore, business risk manager, said: "It appears somebody has gone to great lengths to substitute the Aftershock with something of a similar colour.
We don't know why they did it and can't rule out a third party or malicious sabotage.