Image caption 400,000 lorries use the port of Holyhead every year - just one of several major routes to Ireland from Wales

Welsh ports have been made to sign gagging clauses as part of Brexit discussions with the UK government.

In a letter to the assembly's Brexit committee, UK minister Robin Walker said it was standard practice for "some conversations on confidential terms".

He said non-disclosure agreements made it easier for ports to share commercially sensitive information during Brexit preparations.

Plaid Cymru said the claim was "beyond laughable".

The party's Brexit spokesman in the assembly, Steffan Lewis, added: "Why would any port need to sign an NDA in order to stop itself releasing information that could harm its own interests?"

But in the letter to the Senedd committee, the UK minister said the approach "allows a more free exchange of views about options and potential impacts in advance of there being settled policy".

He added: "It is anticipated that NDAs will be a diminishing feature of these steering groups as an increasing amount of information is made available generally."

A member of the UK government's 'Welsh Ports and Airports Steering Group' confirmed they were "requested" to sign an NDA.

The source added that it prevented "speculation because some of what is discussed will be discounted" and that "nothing that I've heard suggests Armageddon".

The Welsh Government said it had not "subjected any individual or organisation to an NDA".

On Monday, the assembly's Brexit Committee will release a report saying that a 'no-deal' Brexit would pose a "serious threat" to the ports sector in Wales.

The committee's chair David Rees AM said: "What we found is that there needs to be a step-change in Welsh Government activity to support the sector prepare for a no-deal Brexit.

"If our worst fears of new delays and checks at Welsh ports like Holyhead and Fishguard are realised, Wales will need detailed plans to manage the fallout.

"That is why we were calling on the Welsh Government to publish details of any traffic management contingency plans it has, including outlining what new infrastructure spending may be required."

Image caption Setting sail from Pembroke Dock - but what happens next for Welsh ports?

Welsh Government officials said its position was clear and no-deal "should be unthinkable".

"The UK government must ensure that the UK as a whole participates in the single market and a customs union," said a spokesman.

"That would mean no new barriers to trade and no new customs infrastructure at our ports. These matters remain entirely the responsibility of the UK government "

EU leaders signed off on the prime minister's Brexit deal in a specially arranged summit in Brussels on Sunday.

But the potential of leaving the EU without a deal remains because there is no guarantee that Theresa May will be able to get enough support for the deal in the House of Commons.

The vote in Parliament is expected to happen in early December.