Taking a sharp swipe at the Prime Minister, Welsh First Minister Carwyn Jones said he had “never seen a UK Government so unprepared for anything”.
But Welsh Tory leader Andrew RT Davies backed UK ministers, maintaining they had a “serious plan”.
Inadvertently backing the Tories’ Brexit proposals, Ukip’s Neil Hamilton said Britain had “masses to gain” from Brexit as 85 per cent of the world economy was outside the European Union.
Mr Hamilton, leader of Ukip’s group in the Welsh Assembly added: “We can do our own trade deals with the rest of the world.”
The debate comes after the latest Welsh Political Barometer put the Conservatives at an unprecedented 41 per cent, above Labour at 35 per cent and Plaid Cymru at 11 per cent.
In a bid to boost her party’s support, Plaid Cymru’s Leanne Wood said she would ensure Wales’ voice was heard as her party had a “positive Brexit plan”.
Ms Wood claimed her party were focused on “protecting the 200,000 jobs in Wales that rely on tariff-free access to the EU single market”.
However, Welsh Liberal Democrat leader Mark Williams turned the debate on its head and insisted there had to be a second referendum for people to ratify any deal with the EU.
Mr Williams said: “Nobody knows what Brexit will look like – we believe in giving voters the say on the terms of the deal with a ratification referendum.”
Speaking for two hours on the ITV Wales Election Debate, the party leaders also managed to clash over the appropriate level of taxation.
Defending Labour’s plans to raise taxes on higher earners, Mr Jones said: “Welsh Liberal Democrat leader Mark Williams said there had to be a second referendum for people to ratify any deal with the EU.”