Alexi Stafford was at a friend’s house last month when she made a simple error that would claim her life within hours.

Now, her grieving mother is warning other allergy-sufferers and their parents to remain vigilant about what food they choose to eat.

In a heart-wrenching Facebook post she asked others to share, Florida mum Kellie Travers-Stafford explained how her 15-year-old daughter made one simple, but “fatal choice”.

“Our hearts are broken and we are still in shock. Our whole lives we dedicated to keeping our child safe from one ingredient, peanuts,” she began.

Travers-Stafford described how on 25 June Alexi was at a friend’s house and saw an open package of Chips Ahoy cookies on offer. The teen, from Weston, Florida, had been warned by her parents to stay away from foods containing nuts or traces of, but the family had previously figured out that this particular type of cookie was fine.

But what Alexi didn't realise was that this look-alike biscuit packet actually contained cookies with Reese's Peanut Butter Cups.

"The top flap of the package was pulled back and the packaging was too similar to what we had previously deemed 'safe' to her.

"She ate one cookie of chewy Chips Ahoy thinking it was safe because of the 'red' packaging, only to find out too late that there was an added ingredient.... Reese's peanut butter cups/chips."

Alexi immediately felt that something was wrong.

"She started feeling tingling in her mouth and came straight home. Her condition rapidly deteriorated," her mum described.

"She went into Anaphylactic shock, stopped breathing and went unconscious. We administered two EpiPens while she was conscious and waited on paramedics for what felt like an eternity."

Sadly, paramedics couldn't save her and she died "within one and a half hours of eating the cookie".

Posting a photo of two incredibly similar-looking packets - one containing the peanut chocolates, the other not, Travers-Stafford expressed her frustration.

"As a mother who diligently taught her the ropes of what was okay to ingest and what was not, I feel lost and angry because she knew her limits and was aware of familiar packaging, she knew what 'safe' was.

"A small added indication on the pulled back flap on a familiar red package wasn’t enough to call out to her that there was 'peanut product' in the cookies before it was too late."

The mum asked others to share the post to spread awareness to others.

"I want to share our story with everyone because we want to spread awareness...

"It’s important to us to spread awareness so that this horrible mistake doesn’t happen again."

The post has since been shared more than 70,000 times, including by Allergy & Anaphylaxis Australia.

"Our hearts go out to this family in the US who lost their beautiful daughter, Alexi. We hope they have the love and support they need at this really difficult time," the not-for-profit group wrote.

"The sad reality of food allergy is that you have to check every food pack or ask about ingredients every time. Presuming something is fine to eat is not enough.

"Let this be a lesson for all of us. So many biscuits, snack food and other packaged foods have a variety of flavours and packaging can look very similar/almost the same."