EVERYONE knows there is one rule when it comes to weddings – don’t wear white unless you are the bride.

Some wedding guests were left surprised when the bride’s sister-in-law decided to show up in a white, lacey gown on the big day.

The dress was not only white, but also had beautiful lace detailing and a train.

A photo of the sister-in-law having a photo with her brother, who was the groom, was shared on Reddit, and people blasted her for the ‘faux pas.’

The caption read: “This girl looked absolutely gorgeous, stunning and in every word for perfect.

“But she is NOT the bride.

“You had one job, to keep with one rule and still…”

Many people were shocked with the bridal-esque look being worn to another couple’s day.

One Reddit user commented: “You can not tell me she didn’t buy that at a wedding dress shop! That is a full on wedding dress.”

Another added: “If someone showed up in this to my wedding they’d be kicked out immediately.”

A third person said it had happened to them at their own wedding.

They explained: “My MIL wore a custom made white gown to my wedding. Custom. Made.

“She could have picked any colour. Was even a similar cut to mine.

“She said the only colour that looks good on her is white – her daughter got married this weekend and she wore blue so probably not true.

“She also told me not to worry that she bought a black shawl so we would not ‘clash in pictures’…. her words lol.”

Many hailed the sister-in-law’s dress as being “tacky”.

While the backstory behind the dress hasn’t been confirmed, some brides do like their bridal parties to wear white.

We saw this at Kate Middleton’s royal wedding to Prince William in 2011 with Kate’s sister Pippa Middleton wearing a white, sleek gown.

One internet user said of the Reddit post: “Those pictures make it seem like it was clearly a planned thing that the bride was probably okay with.

“The dress matches the bloke’s ties.

“It sounds like the one posting to the group is making assumptions without actually knowing anything about the wedding itself.”

Another added: “Looks like that’s the MOH (maid of honour) or a bridesmaid and if it is they would’ve gotten the OK from the bride.

“I know it’s not common (though it is getting trendy), but I went to a wedding as a kid and all the bridesmaids wore off white.”

Whether the sister-in-law was a bridesmaid or not, the dress has certainly divided opinion.

Let us know your thoughts in the comments below.