The European Union’s chief Brexit negotiator, Michel Barnier, and the Irish prime minister, Leo Varadkar, have warned that a united bloc – “team 27” – will give the UK a stark challenge in the next phase of Brexit negotiations.

Speaking at a joint press conference in Dublin on Monday, both men said EU solidarity would continue to protect members’ interests in “very challenging” talks with London over a trade deal that are supposed to conclude by the end of this year.

The veiled warning came as Downing Street prepared to celebrate the UK’s departure from the EU on Friday.

“We’ll say goodbye to an old friend embarking on an adventure,” said Varadkar. “We hope it works out for them. But if it does not, there will always be a seat kept for them at the table.”

Barnier highlighted the risk of economic disruption if negotiators fail to clinch a deal. “If we have no agreement, it will not be business as usual and the status quo, we have to face the risk of a cliff edge, in particular for trade.”

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The show of solidarity followed combative comments earlier by Varadkar in a BBC interview in which he said the EU was likely to have the upper hand against London and could use leverage over the financial industry to extract concessions on fishing.

Barnier, who is due to deliver a speech in Belfast, said the united front that delivered a withdrawal agreement acceptable to Brussels, Dublin and other members would continue in Brexit’s next phase. “Brexit really showed, we are all part of a family,” he said. “Brexit will not go away. We have important work ahead of us.”

He said the bloc would closely monitor implementation of the Northern Ireland protocol – another veiled warning to Downing Street that there will need to be checks on goods crossing between Northern Ireland and the rest of the UK.

Varadkar, who has sought to remind Irish voters about Brexit in an uphill election campaign for his ruling Fine Gael party, said Ireland wished to remain friends with Britain but would remain part of “team EU”.

In the BBC interview, the taoiseach warned Boris Johnson that divergence from Brussels standards would make an agreement “a lot harder” and offered a blunt reminder about the disparity in negotiating power.

“The reality of the situation is that 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 million. So if these were two teams up against each other playing football, who do you think has the stronger team? So long as we’re united.”

Varadkar also warned against any UK attempt to get a piecemeal deal with the EU. “When I hear people talk about piecemeal, it sounds a bit like cake and eat. That isn’t something that will fly in Europe.”

Reaching a permanent trade deal by the end of the year was going to be difficult so an extension to the post-31 January transition period remained possible, he said, despite Downing Street ruling that out. “But it’s going to be pretty tough over the next few months … we need to get down to business very quickly trying to get that trade deal.”

Varadkar said the UK might fail to get a trade deal giving its banks access to the EU’s financial services market unless it agreed to let EU boats carry on fishing in British waters.

“If financial services and entertainment, audiovisual, are cut off from the single market, the European market, that will be a very severe blow to the British economy and the south-east, in particular in London,” he said.

“So you may have to make concessions in areas like fishing in order to get concessions from us in areas like financial services and that’s why things tend to be all in the one package.”

Johnson’s official spokesman insisted the UK would be “taking back control of our waters, we have been clear on that” when asked about Varadkar’s comments.

But he did not rule out the idea that fishing rights could be a bargaining chip as part of negotiations to ensure the UK gets a better deal in other areas. “‘It will be for the UK to determine for the best interests of the UK who fishes in those waters,” he said.

In remarks possibly aimed at Irish voters, who have backed the government’s tough stance in Brexit talks, Varadkar accused the UK of misreading the first phase of Brexit in part because many people in Westminster and Britain “don’t understand Ireland”.

Later, Johnson said he disagreed with Varadkar about the difficulty of the timing of the trade talks. He said: “I have to say this is one of those rare occasions where I’m going to be obliged to respectfully disagree with my friend the taoiseach and just say I think we can wrap all this up in the time we’ve got.

“We’ve got till, as you know, the end of the year and we will be doing things very fast, [in a] very friendly and respectful way, and in a way also, I think it’s important to stress, that really ensures we look after the interests of the Republic of Ireland as well.”