A pair of friends needed hospital treatment after being served a cappuccino containing cleaning fluid.
Shelly Kirk and Shai Hussain suffered stomach pains, blistered throats and dizzy spells after drinking the drink bought at the Subway branch on Station Road, Bamber Bridge.
They were told the detergent had been left in the hot drink machine at the branch after it was cleaned and had not been drained before serving drinks.
Now, the pair are considering legal action after the manager admitted they had drunk a detergent.
Mrs Kirk, 32, of Bamber Bridge, said: “It was at the bottom of the cup because the more I drank the stronger it tasted.
“To start with I thought it was just in my head, then it started to taste really strongly of detergent, so we took the drinks back to the counter.
“The girl serving did not believe us to start with and she asked another man to try his before she looked at the machine.”
Mrs Hussain, 32, of Lostock Hall, said she had suffered blistering to her throat after having the drink last Thursday.
She said: “I am seeking legal advice because of how ill I was after drinking it.
“What if we had been a young child or an elderly person?
In an email sent to Mrs Hussain, seen by the Evening Post, the team leader at the branch admitted the pair had drunk a machine “cleaning solution.”
Mrs Hussain said: “I was sent a health and safety information sheet by them and it said to go straight to hospital if you had drunk any of this stuff.”
A spokesman for Subway confirmed it is has launched an investigation into the incident.
She said: “The safety and security of our customers is of paramount importance to the Subway brand.
“We have very strict procedures in place to ensure that our stores, which are all operated by franchisees, serve products safely and to customers’ satisfaction.
“We are in the process of investigating the facts of the case at this particular franchise and when we have established exactly what happened we will take whatever action is necessary to rectify what is an isolated incident.”