Boris Johnson has been warned that his large majority will not help him solve the contradictions at the heart of his Brexit plans, amid attempts by the European Union to head off another crisis at the end of next year.

The prime minister has made clear that he will immediately push on with plans to implement his Brexit deal, ensuring that Britain will leave the EU by the end of January.

However, he is already being warned that his 80-strong majority will not hand him any extra power in his negotiations with Brussels. There are concerns in Europe and Westminster that his commitment not to extend the Brexit transition period beyond 2020 also risks exposing critical parts of Britain’s economy.

Sir Ivan Rogers, Britain’s former envoy to the EU, told the Observer that the time pressures created by Johnson’s commitments could force Britain into accepting major concessions. He also doubted that the size of the parliamentary majority would change the debate with Brussels.

“People always think that, but this is the category error that we always make,” he said. “That somehow the strength of your mandate and majority at home makes the slightest difference to how the other side negotiates.

“It’s not totally a repeat game, but it’s essentially the same methodology as 2017, and [the EU] almost certainly think they have the UK somewhat over a barrel, given the prime minister’s self-imposed very tight time pressure in saying he will under no circumstances extend the transition.”

Rogers added that Johnson had not resolved a central contradiction in his plans – that he appeared to want Britain to diverge from EU rules, while also wanting a comprehensive trade deal in place in just a year’s time.

“It’s because you say you have no intention of remaining aligned that the negotiation will take the time,” he said. “What exactly is the point of Johnson’s Brexit as opposed to [Theresa] May’s if he is actually saying that he wishes to remain aligned? The endless ambiguity and telling all sides what they want to hear ends when you start negotiating. See what he does, not what he says.”

The new president of the European commission, Ursula von der Leyen, conceded on Friday that negotiating a future relationship in such a tight time frame would be “very challenging”. She said the negotiations would instead have to prioritise key EU issues, such as the trade in goods and fisheries, and leave others for after 2020.

That could leave arrangements for the UK’s financial services sector and the landing rights of British air carriers out of an initial deal. Such a staged approach would be unwelcome in Downing Street and would, in itself, be difficult to complete.

Brussels is already grappling with the problem posed by Johnson’s position and the short time-frame. The Observer has learned that EU leaders could yet take the initiative and request an extension to the Brexit transition period from the UK.

It is understood initial discussions have taken place in Brussels about the EU taking the initiative and asking for its own extension from the British government. It is likely that the bloc would also need to sweeten the offer of an extension by minimising the costs the UK would face. Sources suggest that such a move might offer Johnson a better chance of gaining cabinet approval, and avoid a cliff-edge exit from the EU’s structures next 31 December, including the imposition of tariffs.

Johnson's withdrawal agreement stipulates the transition can be extended by “one or two years” but must be agreed before next July.

The development highlights the difficulties facing the negotiators as they prepare for talks. The Dutch prime minister, Mark Rutte, said on Friday he feared the negotiations would be far tougher than those over the withdrawal agreement.

Many Tory MPs believe Johnson’s large victory will allow him to ignore the hardline European Research Group of MPs, which includes Jacob Rees-Mogg. Photograph: Ian Walton/Reuters

The debate over the future EU trade deal within the new-look Tory party, and how close MPs want Britain to remain to the bloc, is also unpredictable. There are genuine concerns inside the prime minister’s team that the size of the new Conservative intake means some MPs are unknown quantities. It has also made it hard to calculate the balance in terms of their positions and aims for Brexit, beyond support for the manifesto pledge to formally leave the EU by the end of January.

Many Tory MPs believe Johnson’s large victory will allow him to ignore the hardline European Research Group of MPs, which includes Jacob Rees-Mogg, which has pushed for a hard Brexit that would see the UK drift away from EU rules and standards. However, it is unclear how strong the ERG will be in the new parliament.