Leo Varadkar said the bloc would have the “stronger team” because of its sheer size and influence globally.
He also warned that the EU would not accept a “piecemeal” trade deal where the UK had rights without obligations.
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“The European Union is a union of 31 member states. The UK is only one country. And we have a population and a market of million people, ”the Taoiseach told the BBC.
“The UK, it’s about 139 million. So if these were two teams up against each other playing football, who do you think has the stronger team? ”
Mr Varadkar is meeting with the EU’s chief negotiator Michel Barnier in Dublin in Monday. It comes amid a general election campaign in Ireland.
While the Taoiseach’s approach to Brexit negotiation was widely supported in Ireland, his party has seen its popularity fall since the issue came out of the spotlight.
Britain’s government says it wants a strong free trade agreement that would guarantee it zero tariffs and quotas with the EU market.
But Boris Johnson has set a strict time limit of the end of the year for the signing of a deal, and also laid down a number of red lines that could prove difficult .
Mr Johnson’s government has been reticent to sign up to a full slate of “level playing field” provisions that would strongly link the UK into EU rules in the future.