The EU "will not be rushed" on a trade deal with the UK after Brexit, according to Ireland's deputy PM.
Boris Johnson says a deal can be agreed by the end of 2020 and has included a pledge in his Brexit bill not to extend any transition period to secure one.
But Simon Coveney says it is "probably going to take longer than a year".
Security Minister Brandon Lewis defended Mr Johnson's deadline, saying he had a "strong record of getting things done".
After the UK leaves the EU on 31 January, it will enter an 11-month transition period, where it will largely follow EU rules but will not have any representation in the bloc's institutions.
This period will come to an end on 31 December and Mr Johnson has ruled out extending it any further if a deal on the future relationship between the UK and EU has not been agreed.
The promise is included in the prime minister's Brexit bill, which was voted through by MPs earlier this week and will now head to the House of Lords before becoming law.
But opposition parties have raised concerns about the hard deadline, saying it creates another way of the UK leaving without a deal.
Speaking on the BBC's Andrew Marr programme, Mr Coveney said he accepted the UK was leaving the EU at the end of January, and he hoped for the future deal to "achieve the closest possible relationship" between the two sides.
But he warned there was "no way of the UK... maintaining the same relationship we have today while outside the European Union," adding: "That is the reality of Brexit, I'm afraid."
Mr Coveney said Mr Johnson had "set a very ambitious timetable" in his bill.
"Just because a British parliament decides that British law says something doesn't mean that law applies to the other 27 countries of the European Union," he added.
"The European Union will approach this on the basis of getting the best deal possible, a fair and balanced deal, to ensure the UK and the EU can interact as friends in the future.
"But the EU will not be rushed on this just because Britain passes law."
The deputy prime minister (Tanaiste) said the EU had "constantly warned [Mr Johnson's] timeframe is ambitious, if not unrealistic".
"From an EU perspective, we will try to approach all of these really important and sensitive areas with a sense of partnership and friendship.
"But at the same time, they are complex... [and] in my view, it is probably going to take longer than a year. But we will have to wait and see."
Government minister Mr Lewis admitted the negotiations would be difficult, but he disagreed with Mr Coveney's assessment of the timetable.
"I think we can do it," he told Andrew Marr. "I think it can be done, not just because both parties… are committed to doing it, and want to do it, but we are a country that has already got a known pattern of work with the EU.
"Therefore getting a holistic agreement in the next 12 months is achievable".
Mr Coveney's comments followed a speech by new European Commission President Ursula von der Leyen earlier this week, saying it would be "impossible" to reach a comprehensive trade deal by the end of 2020.
She warned that without an extension to the transition period beyond 2020 "you cannot expect to agree every single aspect of our new partnership". She called the deadline "very tight".
Mrs von der Leyen, a former German defence minister, took over from Jean-Claude Juncker at the start of December. She met Mr Johnson for talks in London last week.