Air Canada has issued an apology after two passengers were told to sit in poorly cleaned seats that had been covered in vomit on a previous flight.

Details of the incident, which occurred on board a flight from Las Vegas to Montreal on August 26, were posted online by passenger Susan Benson.

"There was a bit of a foul smell but we didn't know at first what the problem was. Apparently, on the previous flight someone had vomited in that area. Air Canada attempted a quick cleanup before boarding but clearly wasn't able to do a thorough clean," Benson wrote in a Facebook post published August 29.

"They placed coffee grinds in the seat pouch and sprayed perfume to mask the smell. When the clearly upset passengers tried to explain to the flight attendant that the seat and seatbelt were wet and there was still visible vomit residue in their area, the flight attendant was very apologetic but explained that the flight was full and there was nothing they could do," she added.

The passengers and crew "argued back-and-forth for several minutes," wrote Benson, before a supervisor came over and reiterated that the passengers would have to stay in the vomit-covered seats as the flight was full.

Benson goes on to relate how a pilot then came down the plane to speak to the passengers, telling them that "they could leave the plane... and organize flights on their own dime, or they would be escorted off the plane by security and placed on a no fly list!"

The explanation was that they had been "rude" to the flight attendant, but Benson disputes this.

"They were certainly not! They were upset and firm, but not rude!" she wrote.

Even though a fellow passenger tried to explain the situation, the pair were then escorted from the plane by security.

"For what? Refusing to sit in vomit for five hours!," wrote Benson, who said the airline "literally expects" its passengers "to sit in vomit or be escorted off the plane and placed on a no-fly list!"

She then conceded that she didn't know whether the passengers were in fact placed on a no-fly list.

"I am ashamed to be a Canadian and ashamed of Air Canada," she wrote. "Shame on you Air Canada! shame on you!"

CNN has contacted Benson for further comment.

Air Canada told CNN in a statement that it had issued an apology to the customers "as they clearly did not receive the standard of care to which they were entitled."

"We are reviewing this serious matter internally and have followed up with the customers directly as our operating procedures were not followed correctly in this instance," the statement continued.

"We remain in contact with them about this matter," the airline added.

This is not the first incident involving bodily fluids on planes in recent months.

In June, Habib Battah was flying from Paris to Toronto on Air France when, shortly after takeoff, he noticed a strange smell coming from the footwell under his and his wife's seats.

"It smelled like manure," he told CNN.

Battah then noticed a wet stain on the floor. Staff provided him with cleaning wipes, and as he used them to rub at the stain, they kept turning red.

A flight attendant informed her coworkers, and the captain radioed Paris, asking what the stain under seats 30A and 30B was.

The news came back from Air France HQ: It was human blood. The previous day, a passenger had suffered what Battah said the crew described as a "hemorrhage."

Three days after his flight, Battah was called by Air France and told that the blood had been mixed with feces.

Air France told CNN it "understands and regrets the inconvenience caused by this situation" and that it was in touch with Battah.