Tessa Munt said new regulations from Brussels will mean manufacturers will be able to call their fruit spreads jam even if they are only 50% sugar.
Ms Munt, who is parliamentary private secretary to Vince Cable, said this will mean they will be able to sell a "gloopy sludge" that resembles nothing like the traditional British staple.
She told MPs in a Westminster Hall debate that rules dating back to research in the 1920s meant that jam had to be at least 60% sugar to retain its gel-like quality.
Consumers would be left confused if producers were able to sell products labelled as jam when they were only 50% sugar as their consistency would be similar to inferior European fruit spreads that often "tasted like mud", MPs heard.
Ms Munt said: "I am concerned that this debate will herald what will turn out to be the end of the traditional British breakfast as we know it.
"We all know what we expect when we go to the supermarket - something that is of beautiful quality, beautiful colour and it has a shelf life of about a year.
"By reducing the percentage of total sugar, the characteristic gel-like consistency of jams and marmalade will be lost. The result will be an homogenised spreadable sludge that does not bear any resemblance to what we know in England and what we enjoy as British jam.
"With a 60% sugar content, the colours of sweet preserves are bright and characteristic of the fruit used to make the product. The fruit is fantastic.
"A lower percentage produces products with darker, muddier colours which may affect consumer confidence in the British product.
"In addition, if the consistency lacks the characteristic gel and is instead more like a fruit spread or a fruit butter, consumer confidence in the properties of jam and similar products may be lost."
However, a Defra spokesperson said: "Reducing the minimum sugar content in jam from 60% to 50% will help British producers - large and small - to trade more easily across the world, boosting our economy and allowing jam-lovers everywhere to enjoy delicious British jam."