Carwyn Jones: Trade tariffs a red line to Brexit deal
The first minister will not support any Brexit deal if it means trade tariffs on goods produced in Wales.
Carwyn Jones will meet Theresa May in London on Monday as part of talks between the leaders of each part of the United Kingdom.
Mr Jones told BBC Wales Sunday Politics programme the re-introduction of tariffs was a red line to any deal.
The UK government department in charge of Brexit said it wanted the "best deal for Britain".
A Department for Exiting the European Union (DExEU) spokeswoman said this would include allowing UK companies to trade with the single market in goods and services.
The prime minister has previously said she wants the Welsh Government to be fully engaged in talks.
Ahead of Monday's meeting Mr Jones told BBC Wales companies based in Wales should not have any financial barriers to dealing with the European market.
He said Theresa May's negotiating position with EU leaders would be more difficult if she didn't have the support of every part of the UK.
He previously warned of the "real dangers" Welsh firms could face tariff barriers post-Brexit and raised the issue with Mrs May during her visit to Cardiff Bay in July.
Mr Jones also reiterated that there needed to be changes to the free movement of people from the EU after the June vote.
He said doctors, nurses and company directors must not be prevented from coming to work in Wales, and did not rule out a work permit system.
A DExEU spokeswoman said: "The government is determined that the interests of all parts of the United Kingdom will be taken into account as we negotiate our exit from the European Union.
"We will continue to work closely with the devolved administrations ahead of the negotiations."