Fianna Fáil leader Micheál Martin has today accused the Government of lacking "urgency and ambition" in its response to Brexit.
He has called for the creation of an "urgent national plan" to reduce the impact of a hard Brexit - and to open up new markets.
In a speech at the Seán Moylan Commemoration in Co Cork this afternoon, Micheál Martin said the British government's current policy was "a shambles".
The Fianna Fáil leader claimed Brexit is already undermining Irish businesses and communities.
Mr Martin called for an EU-funded programme to help businesses, like the agri-food sector, which are hardest hit by Brexit.
Mr Martin also said this is a challenging moment in our history - he said we need to work to address the fact that the country has become more divided and more unfair than ever after emerging from a deep recession, on top the rising threats from an international situation which could cause deep, long-term damage.
Calling for an urgent national plan in Dublin to deal with the "slow-motion crash", Deputy Martin has also demanded the EU suspend normal State aid rules for worst-hit Irish industries.
"We are not going to join the English in their desire to repeal the 20th century," he said.
"We will not join them in their right-wing ideology of trade rules with no social dimension and no enforceable laws."
Mr Martin said: "Britain has taken the route of a backward-looking nationalism, suspicious of outsiders and committed to the historically false idea that you don't need strong international bodies to secure lasting cooperation and prosperity between nations."
He added: "In the five months since the UK's Brexit vote the only things which are clear are that their policy is a shambles and that it is already causing real damage on this island.
"Brexit is not something which is happening in two years, it is happening now."
Mr Martin said Ireland urgently needs a strategy to mitigate the impact of "the hard Brexit which is already under way".
"The unprecedented decline in Sterling may soon be followed by new barriers to trade," he said.
"We can't stand by and let this slow-motion crash happen."