Sir John Major has warned that a hard border in Ireland will be unavoidable unless Brexiters start to take on board the “collateral damage” that exiting the EU will cause in Northern Ireland.

He said that the words “customs union” had become “toxic” and that efforts to find an alternative solution to the border amounted to “limp promises”.

The Conservative former prime minister said that Brexiters needed to understand that unless the UK stays in a customs union and has regulatory alignment with the EU, checks will be required for food, animals and animal feed by law.

Quick Guide Why is the Irish border a stumbling block for Brexit? Show Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

“If so, a physical border seems unavoidable,” he said. “And, since the border winds through over 300 miles of countryside, this may require a number of border posts to be erected.

“No doubt many goods can be cleared in some invisible, frictionless way – as yet unidentified – but not all. Some, such as animals and animal feed, which cross the border every hour of every day, will probably have to be examined for health and safety reasons in order to avoid infections and diseases.”

His comments in a speech at the Irish embassy in London on Thursday come weeks after he said the British people had “every right” to a second referendum arguing that voters were misled by campaigners.

Major warned that border checks had the potential to “divide communities that are now united” and provide “a focus for protests from fringe groups – either unionists or nationalists”.

He said politicians should be aware of the consequence of that and ask themselves if there were protests would the border need to be protected. “Would security be brought in?” If so, would that prompt “a downward spiral towards violence”?

“I don’t wish to overplay this but, at the very least, it increases the potential for conflict,” he added.

Major was the first prime minister to normalise Anglo-Irish relations but his interventions have lead to virulent attacks by Brexiters.

Major said in his speech that when he and Tony Blair went to Ireland to warn about the implications of Brexit for the whole island he expected opposition from Brexit supporters. “What we didn’t expect was a complete failure of understanding about its impact by leading Brexiters.

“I don’t believe for a moment that the British government wishes to hurt Ireland: far from it. I simply observe that preventing collateral damage to Irish interests doesn’t appear to feature prominently in the concerns of those who press for Brexit.”

Major conceded that the customs union does have downsides but that these are “heavily exaggerated”.

He said the Irish border question was “much more than a political squabble” between Brexiters and remainers, rather it was “a battle for the future direction of a policy that will affect all the people of the UK and the Republic of Ireland”.