Sisters Donna Jones and Victoria Wardley suffer from Prosopagnosia, a bizarre condition which means they are unable to recognise faces - including those of their partners and children.
The condition is so bad that the sisters, from York, are even unable to recognise their own reflections.
Married Victoria can't even recognise her own husband.
She said: "When I see someone's face it's like tunnel vision. I can make out an eye or a nose, but when I try and look at a whole face it just doesn't work.
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"It's like a blank canvas on someone's head.
"People will come up to me in the street who I've known for years, but until they introduce themselves I have no idea who they are.
"I'm not really sure what I look like, and I couldn't describe my husband to you either.
"We rarely take any pictures because there's no point - we'd have no idea who was in the photo."
Victoria and Donna discovered their condition when their family doctor recognised their symptoms.
Victoria, who works as a dog groomer, can't even recognise the faces of the pooches she preens.
She said: "I've never really thought I had something wrong with me to be honest, I just always thought I wasn't very good at remembering people.
"I only realised it was an issue a few years ago when I was working in a coffee shop. My doctor used to come in everyday and, of course, I never recognised him.
"Eventually he told me to come in so he could run a couple of tests, and we found out I had prospagnosia.
"I've got an actual diagnosis, but most people just work it out for themselves."
It was thanks to 32-year-old Victoria's discovery that sister Donna realised she too was suffering from the condition.
Donna, 30, said: "When Victoria told me what the doctors had said to her, things started to become clear.
"I'd always thought I just wasn't paying enough attention to people, in a way it was a relief to know that something was wrong.
"I've had incidents where I've gone up to men in supermarkets thinking they were my partner, only to realise I'd grabbed hold of the wrong man!
"I even find it hard to pick out my daughter from a crowd. I feel so guilty sometimes - I should know what my own child looks like - but I just find it impossible.
"If she ever went missing, there's no way I'd be able to tell people what she looked like."
The condition means that the sisters rarely go out alone, for fear of loosing each other.
Victoria said: "We could never go abroad on our own together just in case we got split up.
"We've been out shopping before and my sister wondered off - obviously I couldn't find her anywhere.
"I spent ages looking but eventually I had to ask them to put an announcement over the supermarket tannoy.
"I'm sure most people thought I was looking for my lost child or something, not my grown up sister!"
"We always get lost on nights out too.
"We got lost in a nightclub and spent most of the night looking for each other. I'm sure we'd have walked past each other a million times.
"I'm lucky because most of my friends have distinguishing features that I can pick out - they might have long hair, or be really tall. I suppose it was a subconscious decision so I'd be able to recognise them."
Both women also have trouble recognising themselves in a mirror.
Victoria said: "I used to work as a bar manager in a pub and was trying to get to the kitchen with an armful of empties.
"This woman got in my way and I just couldn't get her to move.
"I was getting so annoyed with my hands full that I started shouting at her - it took me a couple of minutes to realise that I was arguing with my own reflection.
"Working at the pub was a nightmare though. We couldn't allow people to start tabs because they'd get away without paying."
Donna has had similar experiences.
She said: "I was walking down a corridor at work when this woman ended up blocking my path.
"We started doing that awkward dance when one of you moves aside only to find that the other has moved in the same direction.
"We were side stepping about for ages until I realised that I was trying to walk into a mirror."
The sisters have however found upsides to their problems.
Victoria said: "Our condition means we rarely get embarrassed. If I do something silly I don't care because I'll never know it if I see them again."
Dr Sarah Bate from the Centre for Face Processing Disorders at Bournemouth University has carried out research into Prosopagnosia.
She said, "Prosopagnosia or face blindness is a cognitive condition characterised by a selective impairment in face recognition.
"Very rarely some people acquire the condition following neurological trauma, but we've recently become aware that many more people have a developmental form of prosopagnosia.
"These people have never suffered any neurological damage, and appear to have simply failed to develop the visual mechanisms that are required for face processing.
Sometimes the condition appears to run in families, and often people report other first-degree relatives who also appear to be poor with faces.
"Recent estimates suggest as many as two percent of the population (that's one in 50 people) have a degree of face blindness, yet public awareness of the condition remains low."