Boris Johnson’s plan to make it illegal for the government to extend the Brexit transition period beyond 11 months has been described as “strange” by Ireland’s deputy prime minister, as Brussels prepares to limit the scope of the coming negotiations.

Simon Coveney said it amounted to the “UK deciding to tie itself in terms of options”, while the director general for trade in the European commission, Sabine Weyand, said the ambition of any deal would need to be pared back.

When the UK leaves the EU on 31 January, difficult and complex negotiations over the future relationship will begin. The country will remain in the single market and customs union until 31 December 2020, with an extension of the arrangement permitted for up to two years if it is both found necessary and agreed before 1 July.

The government has said, however, it will legislate to prevent such an extension.

Why does Boris Johnson want to prevent a transition period extension? Read more

While the move does not prevent the prime minister from changing the law again at a later stage to allow for extra time, government sources suggested the symbolic legislation will focus minds in Brussels.

According to a leaked diplomatic note obtained by the Guardian, the commission and member states have concluded that the UK’s “negotiation commitment doesn’t afford much space for broader ambitions” beyond a “basic free trade agreement”.

The commission is working on a “basic package with a basic free trade agreement for goods linked to conditions of level playing field, governance, external and internal security and fisheries”, according to the document.

Coveney said the history of the Brexit negotiations over the last three years suggested the UK did not prosper when false deadlines were drawn.

“Nobody is forcing the UK to apply for an extended transition period but they have the option to do it if they want to up until the middle of next summer, and what Boris Johnson is doing is essentially ruling out an option that was put into the withdrawal agreement for Britain,” he said.

“We all know that the negotiations post a British exit from the EU is going to be very complicated. It’s going to deal with multiple areas, not just a free trade agreement. It has also got to deal with security, data, fishing; there are multiple sectors which will require detailed negotiations.

“The EU hasn’t missed a deadline yet. It has been the UK that has missed deadlines in the past. I just think if we’ve learned anything from the first round of Brexit negotiations … [it] is that we shouldn’t be closing off options.”

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Speaking at an event organised by the European Policy Centre thinktank in Brussels, Weyand, who was the EU’s deputy chief negotiator, echoed comments made by the European commission president, Ursula von der Leyen, who warned last week that 11 months would be insufficient to negotiate a comprehensive deal.

Weyand, who is now the lead official in the commission’s trade department, said areas such as trade in goods and access to UK waters for European fishing fleets would be prioritised, as would other areas of law where there was no international fallback position.

“I think, given all the signals we are getting, and now these signals are translated into law, that we are well advised to take seriously that the UK doesn’t intend to go for an extension of the transition, and we need to be prepared for that,” she said. “And that means in the negotiations we have to look at those issues where failure to reach an agreement by end 2020 would lead to a cliff-edge situation.”

The European commission’s executive vice-president, Valdis Dombrovskis, who holds the economy brief within the EU executive, also emphasised that time would be limited.

“It will be, indeed, rather problematic to hold negotiations and reach agreement on a comprehensive trade deal,” he said, adding that the “very rigid timeframe” meant “certain things will be out of reach”.

The EU’s chief negotiator, Michel Barnier, said he would do his best to find agreement on a deal by the end of next year, as he arrived in Strasbourg to brief MEPs on the latest developments.

“We will do the maximum,” Barnier told reporters.

Asked to comment on Johnson’s refusal to extend the talks if needed, he added: “I have no comment to make on that. It is the British choice to choose the procedure it wants.”

Theresa May had asked for the transition period – also known as the implementation period – to allow the UK to prepare for a newly negotiated relationship with the EU. She had sought a two-year period but received 21 months with an option to extend.

The proposed sequence of the talks could lead to permanent solutions on key areas, such as data-sharing, transport, and arrangements for the services sector, being left open until after 2020.

After Johnson had a phone call with Von der Leyen on Tuesday, a Downing Street spokesman said the leaders had “agreed to work together with great energy to agree a future partnership by December 2020”.