Carwyn Jones warns over international Brexit trade deals
New trade deals with countries such as the United States cannot replace the UK's current trading relationship with the EU, the Welsh Government has said.
Ministers in Cardiff accept there are "significant trading opportunities" outside the EU and support the "merits of international trade".
But they still prioritise "full and unfettered access" to the EU's single market and customs union membership.
The UK government said it is "committed to securing a good deal with the EU".
But it added the UK will leave the single market and customs union.
Wales exported £14.6bn worth of goods in 2016, with 61% of those being sold to countries within the EU. In the same year, only 41% of all UK goods were exported to the EU.
In 2015, 35% of the total £1.7bn worth of services sold in Wales went to the EU.
"What's hugely important to us is that we have full and unfettered access to our most important market," said First Minister Carwyn Jones, at the launch of a new paper on future trade policy.
"It's hugely important we get the trading relationship with Europe right and of course we can look at what other opportunities there might be in other parts of the world, but nothing's going to replace Europe as an important market.
"At the moment we have access to that market which means we don't have to pay any tariffs, we don't face any barriers.
"Why would we want to put barriers where none exist?"
A second report looking at the possible impact of Brexit on Wales' largest companies was published by Cardiff Business School on Friday.
One of the authors, Prof Max Munday, said their research has found there is no "one size fits all" solution to address the different risk factors faced by companies as a result of Brexit.
"For some firms, yes, possible value-added tariffs are important but for other firms issues such as the labour market implications, non-tariff barriers are most important," he added.
Non-tariff barriers include different regulations, standards and subsidies between one country and another.
The report said: "Care should be taken in over-focusing on firms that appear to be large exporters" because in some of the largest exporting sectors "little value is actually added within Wales".
A spokesman for the Department for Exiting the EU said: "We respect the four freedoms of the EU and that is why, as we leave the EU, we are leaving the single market and customs union.
"We are committed to securing a good deal with the EU that works for the whole of the UK including Wales, through a bold and ambitious future economic partnership."