If the UK defaults to WTO trade terms, the GMB says duties will be paid on many household items, including butter and cheese.
In a report by report by Acuity Analysis commissioned by GMB, own brand butter (250g) could rise 28% from £1.50 to £1.92 ($1.90 to $2.44). Cheese (own brand mature Cheddar, 460g) would rise by a similar amount, from £2.30 to £2.92 ($2.92 to $3.71), an increase of 26.9%.
According to the document, milk prices would be unaffected.
Hikes on other imported goods including confectionery, fruit and vegetables range from 8.8% for watermelon through to 17.3% for honey, with some items, for example Tesco finest Pinot Noir 70cl, rising 32%.
The document says rising costs could mean job losses, as companies try to cut costs elsewhere, and higher consumer prices as companies pass on all or some of the increase in costs onto their customers.
It notes that 27% of all food consumed within the UK originates from elsewhere in the EU and in 2015, the UK imported €31bn of food and drink from the EU, with dairy high up on the list from Ireland, France and Germany.
Tim Roache, GMB general secretary, said, "Tory leadership contenders who casually throw around the idea of 'no deal' Brexit are completely ignoring what that reality would mean for working people.
“The prices of household essentials will go through the roof if hardliners like Raab and Johnson get their way, but why let people's actual lives get in the way of personal ambition in the Tory Party? Either they're negligent in understanding what leaving on WTO terms means or they just don't care.
“If the Tory Party choose a leader prepared to walk us off a Brexit cliff edge, our country will live with the economic consequences of that for a generation. They won't be forgiven easily for that at the ballot box."