Its advertising campaign highlights how luxurious its food offering is with the line: 'This isn't just any food, it's M&S food.'

However, one customer has challenged the high street brand, by alleging that Marks & Spencer's fancy biscuits are just traditional custard creams covered in chocolate.

Peter Marshall, 25, from Ramsey in the Isle of Man, made the claim in a post on Instagram after making his 'discovery' on dipping his chocolate-covered biscuit in a cup of hot milk.

But the 25-year-old from the Isle of Man said he has been left more than a little surprised by the interest his post about the biscuits on Instagram has garnered

Peter Marshall said he discovered the fancy M&S biscuits were custard creams after he dunked one in a cup of hot milk and the chocolate melted away

He posted a photo of the soggy custard cream with a box of M&S 'Extremely Chocolatey' biscuits - which sell for around £6 a box - in the background and the caption: 'Marks and Sparks are a bunch of cheapskates.'

Peter told MailOnline: 'I was dunking biscuits into hot milk and as the chocolate melted away I noticed that the base of one of the biscuits was actually a custard cream.

‘I’ve always held M&S in quite high regard when it comes to food, and finding out that they were using a more common biscuit as a base was a bit of a shock.’

Mr Marshall, from the Isle of Man, said: ‘I was on my own when it happened. I did tell people later but they didn’t really think it was that interesting – I was surprised at that to be honest,’ he said.

So it was a shock for Peter to later garner so much interest. 'It's very odd,' he said.

'The post didn't really garner much of a response even amongst my friends, so, days later, getting so much public interest is pretty bizarre.

'I guess people have strong feelings about biscuits here in the UK. And strong feelings about what should and should't make it into the news.'

Some said that they felt cheated, others laughed and one suggested that far from being a scandal, finding a custard cream biscuit under a chocolate coating was something of a win-win situation.

Peter said he would not be making a formal complaint and the episode had not put him off buying Marks & Spencer biscuits in the future.

In fact, M&S insists that there is no scandal at all, but rather a simple misunderstanding. A spokesman for M&S said it was a 'one-off' caused by the factory running out of the normal moulds used for making the high-end biscuits- and that the recipe was still 'luxury'.

She said: ‘The biscuits used in the selection box are made with a luxury biscuit ingredient mix and are made using the custard cream mould before being covered in chocolate.

‘They may be in a custard cream mould, but they are not custard cream biscuits. What's happened is that we used the same mould as it's the same shape and size.

'It doesn't have cream in the middle and the luxury biscuit recipe was still used - it was just made in the same mould. We are not fooling the nation and there is no untoward biscuit activity going on.'

But the customer said it would not put him off buying biscuits from the brand in the future

Plenty of people will have no doubt been scoffing the special M&S treats over the festive period, but Peter Marshall claims they are traditional biscuits in disguise - the high street giant is yet to comment

The custard cream is thought to have originated in 1908, with two crisp biscuits sandwiching a butter cream filling.

Some suggest that the famous Victorian era biscuit manufacturer Huntley & Palmer created the first versions at its factory in Reading. However, credit may belong to Crawfords, of Edinburgh, which was the best known custard cream brand for generations of families.

Today many supermarkets produce own-brand versions, with the butter cream replaced by cheaper mass produced fats. Variations include lemon, orange, chocolate, strawberry, coffee and coconut flavours.