Crashing out of the EU without a deal would risk breaking up the United Kingdom, the former president of the European council has warned.

Herman Van Rompuy, the former Belgian prime minister who was council president until 2014, told the Observer that he believed the threat of a no-deal Brexit was a new “operation fear” tactic being used by the government. But he said it would not work with the EU and warned that such an outcome would end up creating new pressures over Scottish independence.

“The no-deal issue is not just a problem for the UK or Brussels,” he said. “It is also an existential threat to the UK itself. One can imagine that a no deal will have a big impact and cause concern in some of the regions. Speaking of Scotland, it could have consequences for them and others.”

He added: “We could end up with a situation in which the EU27 becomes more united and a United Kingdom less united. This talk about a ‘no deal’ is the kind of nationalist rhetoric that belongs to another era.”

Nicola Sturgeon, the Scottish first minister, said last week that a no-deal Brexit would be an “unmitigated disaster”. David Lidington, the Cabinet Office minister, has already attempted to calm fears that Scotland’s place in the union could be jeopardised by crashing out of the EU with no deal. Scotland voted for the UK staying in the EU by 62% to 38% and Sturgeon has pledged to outline her thinking on independence in October. But polling experts say there has been little sign of an increase in support for independence as a result of Brexit.

While Sturgeon is not expected to make the case for backing a new independence referendum soon, Van Rompuy’s warning is a sign of the volatility that could be unleashed by leaving the EU without a deal, a prospect which last week saw the Conservative party descending further into civil war.

Dozens of Tory MPs are now said to be willing to take whatever action is necessary to block such an outcome. But the row is set to rage for weeks, with the government preparing to release dozens more “technical notices” on how businesses and consumers should cope in the event of a no-deal Brexit.

Labour remains under pressure over its ambiguous Brexit stance, with a new YouGov poll suggesting that the voters it would need to win a parliamentary majority are principally pro-European. Among voters who did not back Labour last year but said they might or would seriously consider doing so, 68% backed EU membership. The 10,000-strong poll for the People’s Vote campaign found that 59% of these voters wanted a new referendum on Brexit, while 27% did not.

Van Rompuy, the first person to hold the post of European council president, said a no-deal Brexit might lead to another election in Britain. “If there is no House of Commons support for no deal, then you are very close to new elections,” he said. “If you have new elections, then article 50 [the legal process for Britain’s EU exit] will have to be postponed, because it will not be clear that you will have a government – or a government with a programme.”

He suggested Theresa May’s hopes of Britain in effect remaining in a single market for goods but not services might not be accepted by the EU. “It is rather difficult to make a distinction between goods and services. We are living in a new economy where there is a mix of goods and services for the same kinds of products. Saying that we will have a customs union, or even going further with a single market for goods, and completely separate it from a single market in services – this is what [chief EU negotiator Michel] Barnier called unworkable.”

He criticised Liam Fox, the pro-Brexit international trade secretary, for suggesting a no-deal outcome was now more likely than not. “I don’t like this talk of a ‘50/50’ or ‘60/40’ chances,” Van Rompuy said.

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“I know there are so-called tactical considerations about all these discussions on the chances of the no deal. Some want to put pressure on parts of the Conservative party. It is an ‘operation fear’. Those threats will not work vis-à-vis the European Union … I cannot imagine that a British prime minister or a responsible British government is even considering seriously a no deal, playing with the economic future of the country and its people.”

A Downing Street spokeswoman said: “We have always said the United Kingdom would continue to thrive in the event of a no-deal Brexit. But we are confident of getting a good deal – one that delivers for every part of the United Kingdom and takes back control of our money, laws and our borders. That is what this government will deliver.”