Irish Prime Minister Leo Varadkar has suggested the EU will be the "stronger team" in post-Brexit trade talks with the UK.
Comparing negotiations to a football match, he suggested to the BBC that the EU would be at an advantage due to its larger population and market.
The taoiseach said he did not think the UK had "yet come to terms with the fact it's now a small country".
Boris Johnson said he would be able to "wrap this all up" by the end of 2020.
Mr Varadkar held talks with EU chief negotiator Michel Barnier on Monday.
Speaking after the meeting, Mr Barnier told reporters the two sides faced "the risk of a cliff edge" if trade terms were not agreed by the end of the post-Brexit transition period in December.
He cautioned that a "very short time" remained to "rebuild" the UK-EU relationship.
In an interview earlier with BBC political editor Laura Kuenssberg, Mr Varadkar also said striking a deal in this timeframe was possible but would be "difficult".
He pledged the EU would not be "dragging its feet," but added: "My assessment is that it is more likely that we will need an extension in order to finalise a free trade agreement and future economic partnership than not need it."
Mr Johnson, however, said he had to "respectfully disagree" with his Irish counterpart's doubts, insisting a deal can be reached "in the time we've got".
The UK PM added: "We've got until the end of the year, but we will be doing things very fast, and in a very friendly, respectful way."
Mr Johnson has insisted he is not open to any extension.
Meanwhile, it has been confirmed that from the UK side, trade talks will be led by a 40-person "task force" headed by the PM's Europe adviser David Frost.
Mr Varadkar, the leader of the Fine Gael party, is fighting his first election campaign as taoiseach. Ireland heads to the polls on 8 February.
He told the BBC: "The European Union is a union of 27 member states. The UK is only one country. And we have a population and a market of 450 million people.
"The UK, it's about 60[m]. So if these were two teams up against each other playing football, who do you think has the stronger team?"
He also cautioned the UK against trying to negotiate individual deals covering different sectors of the economy.
"The final deal, the new relationship will have to be comprehensive," he said.
"When I hear people talk about piecemeal, it sounds a bit like cake and eat," he said, adding: "That isn't something that will fly in Europe."
"You may have to make concessions in areas like fishing in order to get concessions from us in areas like financial services."
'Written in law'
Mr Varadkar said there was "genuine concern" across Europe that the UK would seek to "undercut" EU standards after Brexit.
"When I meet Prime Minister Johnson he says, no, absolutely not - that's not the kind of United Kingdom that I want to lead as prime minister."
But he added: "We want that written down in law, we want that in a treaty."
Mr Varadkar said both sides would have to agree a "common set of minimum standards" for an agreement to be possible.
But this is likely to be a contentious area of talks, with British ministers having insisted the UK should have the right to move away from EU regulations.
Another potential flashpoint is likely to be access to fishing waters, which both sides have pledged to sort out before the end of June.
Leaked slides from an EU presentation last Friday said the bloc would be aiming for the same level of access to British fishing stocks it has now, and would not sign a wider trade deal until fishing access has been agreed.
But the UK government insists it will "take back control" of its waters.
A leaked slide presentation from a meeting last Friday has revealed more of the EU's objectives in the upcoming trade talks.
Diplomats from national governments agreed that commitments by the UK to maintaining a level playing field - i.e. not undercutting other EU nations for competitive advantage - are a "precondition" for a deal. There should also be a role for the European Court of Justice in any deal to protect EU law.
The EU will pursue what it calls a "comprehensive approach" to the negotiations and there will be "trade-offs between chapters" i.e. give-and-take across different areas of the deal.
The EU will expect to be treated as a single bloc, so the UK will not, for example, be able to offer something to Germany that it doesn't offer to everyone else. In case of future disputes with the UK, there would be the possibility for "cross-retaliation" where a disagreement in one sector sees the EU retaliating in another.
EU sources say they want to build a relationship with the UK that is balanced and sustainable, where neither side "feels taken for a ride".