Abbie Bradish noticed the 'funny looking dregs' drizzling out of her can when she poured herself a drink.
After it happened a few more times, she filmed a fresh can being poured and noticed the disgusting sludge dropping out onto her sink.
Posting to the Morrisons Facebook page, she shared a video of her find and wrote: 'Hi lovely Morrisons folk.
'I noticed before Christmas when I poured the last bit of my Morrisons diet coke away it had some funny looking dregs in it.
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'I thought no more about it until it happened a few more times. We decided to open a fresh can the other day to make sure it wasn't something that was happening when we drank it and this is the result.
'I had two six packs both the packs did the same (I have 2 of each pack left). I haven't bought any in a while as this has really put me off so I've no idea if your current stock is doing this.'
The minute-long video shows the cola being poured out into a glass before being moved to the sink to pour the remaining contents down the sink.
As the silver can is tipped out, dark brown sludge falls out and sticks onto the sink along with the last remaining fizzy liquid.
A member of the Morrisons social media team responded to her post and wrote: 'Oh no. I am really sorry about this Abbie. It is very unusual and we would like to look into this further. Are you able to return the cans to store at your convenience so we can investigate?'
Bradish responded to say she would be able to bring the can back and Morrisons confirmed that they could offer her money back or an alternative product.
A Morrisons spokesman said: 'We have asked Abbie to bring these into store so that we can investigate further.'