Brendan Howlin, leader Ireland’s Labour Party, warned a “countdown to a hard border” was already underway and demanded Mr Varadkar do more to avoid it.
Mr Howlin said more needed to be done by the Irish Government to prevent a hard border between between the Republic and Northern Ireland.
The 300-mile long invisible partition, which has more border crossings than across the rest of the EU combined, will become the only land barrier between the EU and the UK after Brexit.
And with confusion remaining on how goods will travel freely between the two jurisdictions, Mr Howlin said without more preparation Ireland could face the “worst of both worlds”.
Both Dublin and Brussels have warned from the start of Brexit talks any physical infrastructure – including customs checkpoints or even CCTV cameras – would be vetoed.
But Mr Howlin said the situation demanded more than platitutudes from Dublin as well as Westminster.
He said: “Here we are on the eve of an important June summit and we’ve no landing zone.
“We have further confusion about what the British government want to achieve.
“We are now in danger of having a Hard Brexit and the worst of all worlds.”
He urged the Irish government, of which Labour was a coalition partner from 2011 to 2016, to do more.
He said Fine Gael and its leader Leo Varadkar needed to set out what it will do “in the event that we simply drift into the inevitability of a hard exit of Britain from the European Union”.
He said he fears “an endless fudge until we fun out of time”.
This outburst comes despite assurances from Mr Varadkar earlier in the week a hard border would “never” be accepted on the island, even if Northern Ireland is pulled from the single market and customs union.
Mr Varadkar said: “That will just never happen, ever.
“We’re going to need to see some more movement from London between now and the summit at the end of the month before we can be satisfied.”