Neil Hamilton blasted the white paper published by the two parties as they joined forces to unveil their alternative EU divorce plans.
The Ukip leader in Wales claimed the paper was nothing more than a “white flag of surrender” to Brussels and would leave Theresa May in a disastrous negotiating position with the bloc’s leaders.
First Minister Carwyn Jones and Plaid Cymru leader Leanne Wood claimed their proposal sets out a "comprehensive, credible plan based on solid evidence that centres on Wales’ priorities".
Mr Jones said the plan respected the Welsh vote to leave the EU but would give the UK "full and unfettered access" to the single market.
Ukip's leader in Wales claims Welsh Brexit plans would mean no EU departure
If Theresa May were to take this blueprint to Brussels then the EU would get everything they wanted
“Our plan explains how we can strike a balance between the message the Welsh people gave us with the economic reality that we face,” he said.
Ms Wood said that free movement of people from the Continent is “not a problem” during Monday’s address in London.
According to the paper, 79 per cent of EU migrants aged 16-64 in Wales are in employment.
Speaking to the BBC, Mr Hamilton roared: “It’s not so much a white paper as a white flag of surrender to the EU before negotiations have actually started.
“If Theresa May were to take this blueprint to Brussels then the EU would get everything they wanted.
“It would mean that we weren’t leaving the EU in any meaningful sense at all.”
Nathan Gill, Ukip’s Wales MEP, added that the publication proved “just how out of touch the political elite in Cardiff Bay are with the majority of Welsh voters”.
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“The latest YouGov poll released at the beginning of this month showed that the majority of Welsh voters want full control over immigration,” he said.
“The only way to control the quantity and quality of people coming to Britain is to leave the single market.”
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Mon, January 23, 2017
If you are an EU national, you do not need to show your national ID card or passport when you are travelling from one border-free Schengen EU country to another. The Schengen Area is an area comprising 26 European states that have officially abolished passport and any other type of border control at their mutual borders.
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The Welsh Conservative leader launched a separate attack on Welsh Labour and Plaid Cymru, claiming he would have been happy to participate in a wider cross-party agreement but said he was not invited to the negotiating table.
Andrew Davies said: “We have a disorganised message coming from Wales.
“When the single market was created and the tenets that underpin the single market, this mass movement of people across Europe wasn’t a consideration.
“It’s a consideration today.”