The cost of a family’s weekly shop could rocket by more than £800 a year if the UK leaves the EU without a deal, a major union has warned.

Analysis for the GMB found that the bill for a typical supermarket basket of goods would increase by £15.61 a week – 17 per cent – if Britain was forced to fall back on World Trade Organisation rules, which require tariffs on many goods.

Several candidates for the Conservative leadership, including frontrunner Boris Johnson, have said they are ready to leave the EU without a deal on WTO terms on 31 October if it proves impossible to renegotiate Theresa May’s withdrawal agreement by that time.

Dominic Raab has refused to rule out suspending parliamentary sittings beyond the Halloween Brexit deadline to prevent MPs blocking a no deal.

Under the WTO’s “most favoured nation” rules, the price of a 250g pack of butter would rise by 42p (up 28 per cent). Other increases would include 62p for a 460g block of cheddar(up 29 per cent), 43p for a pack of eight sausages (up 25 per cent), 32p for 2.5kg of potatoes (up 14 per cent) and £2.56 for a bottle of red wine (up 32 per cent), according to Acuity Analysis.

Tariffs in the event of a no-deal Brexit Show all 15 1 /15 Tariffs in the event of a no-deal Brexit Tariffs in the event of a no-deal Brexit Pork There will be tariffs on pork in order to protect British farmers Getty Tariffs in the event of a no-deal Brexit Cheese There will be tariffs in place on some cheeses including €22.10/100kg of cheddar, €19.10/100kg of processed cheese and €18.60/100kg on some blue cheeses Getty Tariffs in the event of a no-deal Brexit Milk There will be no tariffs in place on milk Getty Tariffs in the event of a no-deal Brexit Car Parts There will be no tariffs on car parts imported from Europe PA Tariffs in the event of a no-deal Brexit Cars However finished cars will face tariffs of 10.6% Getty Tariffs in the event of a no-deal Brexit Alcoholic drinks There will be no tariffs on alcoholic drinks - except on some rums due to ingredients used in their distilling process Getty Tariffs in the event of a no-deal Brexit Beef There will be tariffs on beef in order to protect British farmers Getty Tariffs in the event of a no-deal Brexit Fish There will be no tariffs on many types of fish including cod, haddock, salmon and sea bass Getty Tariffs in the event of a no-deal Brexit Fruit and vegetables There will be no tariffs on almost all fruit and vegetables Getty Tariffs in the event of a no-deal Brexit Chocolate There will be no tariffs on chocolate or other cocoa products Getty Tariffs in the event of a no-deal Brexit Poultry There will be tariffs on poultry in order to protect British farmers Getty Tariffs in the event of a no-deal Brexit Ceramics There will be some tariffs in place on ceramis Getty Tariffs in the event of a no-deal Brexit Steel There will be no tariffs on steel Getty Tariffs in the event of a no-deal Brexit Coal There will be no tariffs on coal Getty Tariffs in the event of a no-deal Brexit Lamb/Mutton There will be tariffs on the meat of sheep in order to protect British farmers Getty

Releasing the figures on the eve of GMB’s annual congress in Brighton, the union’s general secretary Tim Roache 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.”

Labour MP Clive Lewis, a supporter of the Best for Britain campaign for a Final Say referendum, said: “These Tory leadership candidates who back a no-deal Brexit have no idea what it means to be working person in the UK.

“The reason they advocate this disastrous route is because they will never be in the difficult situation of having to make ends meet.