The Welsh Tory leader has said David Cameron's EU deal was not "robust" enough for him to vote for the UK to remain a member in June's referendum.
On Monday, Andrew RT Davies revealed he planned to vote to leave the EU.
Speaking on Tuesday, Mr Davies said there were "very good arguments to stay in" but "stronger arguments" to leave.
First Minister Carwyn Jones accused him of asking Welsh farmers receiving £200m of EU subsidies each year to "jump off the edge of a cliff".
Mr Davies told reporters: "I've always said that my instinct has to be out of the European Union as currently constructed.
"I believe that the prime minister negotiated very hard for the United Kingdom and ultimately when he was around the table with the other 27 leaders, obviously he had to make the call as to how he felt the deal was going.
"For me it wasn't the robust deal that I think we should have had.
"I do believe that the deal could have gone further in securing concessions, especially cementing the concessions so that they weren't time limited.
"So on balance - and there are very good arguments to stay in - but I believe there are stronger arguments to come out of the European Union as currently constructed and that's why I stuck to my guns and stuck to my principles and announced yesterday."
He said he would not campaign in the referendum until after the assembly election on 5 May but said it would not be right to go into the election as a party leader without declaring his position.
'Too much money'
Later, during First Minister's Questions, Mr Jones told AMs Welsh farming received "more than £200m worth of direct subsidy every year" from the EU.
He said: "Where is that money going to come from in the future?
"It cannot come from Welsh government, it's too much money for us to be able to afford.
"The reality is that Welsh farmers know where they stand at the moment, and they will resist being invited to jump off the edge of a cliff in the hope there's a net on the other side, as they have been invited to do by the leader of the opposition."
On Monday, the prime minister told MPs that leaving the EU would mean poorer parts of Wales would lose out on European grants that had "made a big difference".