In a footballing analogy, he said the bloc had a “stronger team” because of its far larger population and market.
Varadkar also suggested Boris Johnson may run out of time to secure a trade deal signed before the end of the year when the transition period finishes.
Varadkar, who is fighting a general election, spoke to the BBC before meeting the EU’s chief negotiator, Michel Barnier, in Dublin.
“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,” Varadkar said, The Guardian reported.
“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?”
Johnson has repeatedly ruled out requesting an extension to the transition period, during which the UK abides by EU rules, beyond December 31.
But Varadkar, whose talks with the prime minister were seen as key in securing a breakthrough on the exit pact, questioned whether it would be possible to negotiate a full trade deal in time, saying “it will be difficult to do this”.
He also warned against any attempt by the UK to broker parts of a deal over time with the EU. “When I hear people talk about piecemeal, it sounds a bit like cake and eat,” he said. “That isn’t something that will fly in Europe.”