Brexit could drive up energy bills, power companies have said, because trade barriers threaten to increase the cost of importing gas and electricity across the Channel.

France’s EDF, multinational Unilever and the UK’s energy industry body urged politicians to avoid imposing tariffs or barriers on energy trading across borders.

In a letter to Jean-Claude Juncker, the European commission president, and UK prime minister Theresa May, they said imposing costs on the use of interconnectors – electricity cables and gas pipelines between the UK and its European neighbours – would hit consumers and set back the battle against global warming.

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The free flow of energy across interconnectors was necessary to keep “a level playing field that keeps costs down for consumers and ensures decarbonisation and security of supply,” the groups said. Interconnectors currently account for 6% of British power supplies, but that share is soon expected to rise to a fifth.

The UK is viewed in Brussels as a progressive contributor to EU climate change policy. The letter also voiced concerns that any impact on interconnectors could hit the fight against global warming, because they are seen as useful for balancing energy supplies by guaranteeing a source of power when wind or solar power are not available.

“Any imposition of tariff or non-tariff barriers to the flows of energy across interconnectors would increase the cost of the low-carbon transition and set back action on climate change,” the groups said in the letter, whose signatories included green energy groups RenewableUK and WindEurope.

Imposition of cross-border electricity tariffs also poses a possible existential risk to the Irish single electricity market between Ireland and Northern Ireland, the letter said. Whitehall has drawn up plans to send electricity generators on barges to Northern Ireland should the market collapse as a result of the UK crashing out of the EU without a deal.

The intervention by major energy firms on Tuesday echoes earlier warnings from a House of Lords committee and energy professionals that adding friction to energy trading between the UK and EU could push up household bills. The risk of such a scenario has become much greater with the increased possibility of a no-deal Brexit.

Millions of UK consumers have already been hit by two rounds of price hikes this year caused by rising wholesale electricity and gas costs, so any further costs would put serious pressure on household budgets.

The letter to Juncker and May, sent before an upcoming UN climate summit in Poland, calls for a comprehensive energy and climate change chapter to be prioritised in Brexit negotiations between London and Brussels.

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The UK and EU should cooperate on implementing the Paris climate deal, said the signatories, which also include the British Irish Chamber of Commerce, green energy groups RenewableUK and WindEurope, and sustainable investment group Earth Capital Partners.

Pete Clutton-Brock of the E3G environmental thinktank, which coordinated the letter, said the letter showed UK and EU firms supported strong action on climate change. “A no-deal Brexit scenario on the other hand would lead to an increase in UK energy bills, undermine action on climate change and threaten the supply chains of strategically important industries such as offshore wind, electric vehicles and battery technology,” he said.

A government spokesperson said: “The UK has always been one of the most ambitious and leading countries in the world, including Europe, when it comes to tackling climate change and that commitment is unwavering.

“As set out in the [Brexit] white paper, we will maintain our high standards after we leave the European Union and we will respond in full to this letter in due course.”