Ron Morais told CBC News he picked up the takeaway coffee on the way to work in Fredericton on Monday and did not notice anything wrong until he removed the lid.

“I always take the lid off to get my last sip of coffee. And when I took the lid off, there was a little bit of a surprise in my coffee cup. It was a dead mouse,” he added.

There were also droppings swimming in the dregs of the coffee, Mr Morais claimed, adding that he felt “queasy” as the realisation hit him.

As reports "Armenpress" citing "The Independent", Jason Patuano, the communications manager for the eastern region of McDonald’s in Canada, issued a statement saying food safety was a “top priority” for the chain.

“We take allegations involving cleanliness and sanitation very seriously,” he added.

“Upon learning of this situation, the local franchisee immediately began an investigation.”

An inspection by the Canadian Department of Health on Tuesday said there was “no evidence of pests,” and all food was sealed.

“We are continuing to investigate this isolated incident and have reached out to the customer to obtain the product sample,” a McDonald’s spokesperson said.

Mr Morais said he wanted to know how the mouse got into the cup but was otherwise unconcerned by the incident.

“I mean, hey, you know. I'm here. And I'm in better shape than the mouse,” he told CBC.

The incident came after McDonald’s invited an external camera crew inside their factories for the first time to show consumers exactly what goes into their burgers.

It is hoping to dispel the unappetising rumours about ‘pink slime’, worm meat and other burger fillers that have long haunted the chain.

McDonald's insisted its new turn towards transparency is not linked to business performance. Instead, chief brand manager Kevin Newell told Good Morning America the decision is linked to “making sure our customers truly know the story about McDonald’s food”.