The president of the European council, Donald Tusk, condemned Jeremy Hunt’s “unwise and insulting” comparison of the EU to the Soviet Union, as he piled pressure on the British government to compromise for a Brexit deal.

Tusk also implicitly criticised Theresa May after she accused the EU of not showing her enough respect. “Emotional arguments that stress the issue of dignity sound attractive but they do not facilitate agreement,” he said.

His most withering words were directed, however, to Hunt, the British foreign secretary, who had likened the UK to a prisoner trying to escape the EU, and Brussels’ tactics to those of the Soviet Union.

“Comparing the European Union to the Soviet Union is as unwise as it is insulting,” Tusk said. “The Soviet Union was about prisons and gulags, violence, against citizens and neighbours. The European Union is about freedom and human rights, prosperity and peace, life without fear. As the president of the European council and someone who spent half of my life in the Soviet bloc, I know what I am talking about.”

Tusk, a former Polish prime minister, was jailed for a few days in 1983, for publishing a banned periodical. He lost his job and with his wife was evicted from his home when Poland’s rulers cracked down on opponents in 1981.

In a pointed reference to the novichok poisoning attack in Salisbury, Wiltshire, Tusk said the Soviet spirit was “still alive” – but not in the EU. The union, he said, had been the “first to declare full solidarity with the UK at a critical moment” following the Salisbury attacks.

Referring to accusations of a cyber-attack on the Organisation for the Prohibition of Chemical Weapons, Tusk promised to put cyber security on the agenda of the next EU summit. “On behalf of the EU I want to fully condemn this attack and express our solidarity with those affected.”

Tusk was speaking to journalists, after talks with Ireland’s prime minister, Leo Varadkar, as EU leaders, in Brussels, met to try to break the deadlock over the Irish border.

Varadkar was also meeting the EU’s chief negotiator, Michel Barnier, and members of the European parliament.

The meetings are a chance for the EU to assess next moves on the biggest obstacle to a Brexit deal, as the clock ticks down to a make-or-break EU summit on 17-18 October.

The EU is not moving on its opposition to the British prime minister’s heavily criticised Chequers plan. Tusk said that from the start the EU had offered the UK “a Canada-plus-plus-plus deal, much further reaching on trade, on internal security and foreign policy cooperation”. (This offer, however, falls short of the single market on goods desired by the British government.) “This is a true measure of respect and this offer remains in place,” Tusk said.

Echoing Tusk’s remarks, the taoiseach, Varadkar, said it was time to get down to business. The EU and UK are at loggerheads over the “backstop”, an insurance plan to avoid a hard border on the island of Ireland regardless of the future trading relationship between the EU and UK.

Some sources think the gap between the British and Irish over the backstop could be closing, with May prepared to accept some checks on goods travelling between Britain and Ireland, but away from the border.

The DUP threatened to pull the plug on May and vote against a deal that would place a border in the Irish Sea, but sources say all efforts are being concentrated on persuading opponents that these checks would not be “in the Irish Sea”.

“A lot of this is about language. If the checks are not being done, for example, in transit, on vessels, this is not the border,” said a Dublin source.

Varadkar can expect a show of support from EU politicians, who have long insisted there will be no Brexit divorce deal without agreement on the Irish border.

British and European officials are redrafting rival versions of the backstop – the fallback plan to prevent a hard border returning to the island of Ireland. The backstop would swing into place if the two sides fail to resolve the issue through a trading agreement, the UK’s preferred option.

Well-placed sources say the EU’s new text for the backstop will be “substantially different” from the March one, which incensed May by tying Northern Ireland indefinitely to the EU customs union and single market in the event of no other agreement.

In an attempt to “dedramatise” the issue the EU has revised its backstop plan so that customs, VAT and market-standards checks, can be done at company premises or via trusted-trader schemes, making the presence of EU law in Northern Ireland less obvious.

However, the EU will not compromise on checks on animals, food and plants, fearing the bloc’s safety and welfare standards could be diluted. Northern Ireland would therefore be required to sign up to some EU single market rules and the EU customs union.

Officials are also working to “improve” the link between the backstop and future trade deal, in an attempt to convince the British the backstop would never be activated.

The British are also revising their backstop plan, with the Brexit secretary, Dominic Raab, expected in Brussels in the coming days to present the new offer.

In a sign that there are hopes of a breakthrough to end the impasse, there are expectations in Dublin that the much-anticipated British text will drop its requirement that the backstop be time-limited. The UK backstop plan outlined in June would mean the whole of the UK remaining aligned with the EU customs union for a limited time. But the EU wants a Northern Ireland-only backstop and deems the time limit unacceptable.

Sources have told the Irish Times that the British will switch to an “event-limited” backstop, which would only expire on agreement between the EU and UK on a trading relationship that maintained the open border.