Welsh and Scottish minsters have demanded the UK government reconvene a Brexit-liaison group which has not met for six months.
The joint ministerial committee (JMC) to seek a UK-wide approach to leaving the European Union was meant to meet monthly.
The Welsh Government said it was "unacceptable" it had not met since February.
The UK government said its engagement on Brexit had been "unprecedented".
Plaid Cymru MP Jonathan Edwards called the lack of meetings "unforgiveable", adding there was a risk that Wales' needs would be "ignored and forgotten once again by the British government".
Set up by the UK government the new group, a sub-committee of the JMC that focused on the Brexit negotiations was intended to allow ministers from Wales, Scotland and Northern Ireland to set out their priorities for leaving the EU.
The plan - under an agreement with Prime Minister Theresa May last October - was to ensure "outcomes for all four governments are secured" in the negotiations.
The cross-nations group met four times between November and February, but it has not met since March's meeting was postponed because of elections in Northern Ireland.
A House of Commons research paper said until a government for Northern Ireland is agreed, discussions on Brexit are likely to be held directly between the UK government and the Scottish and Welsh governments individually.
UK Brexit minister Robin Walker said it was anticipated there would be "regular and sustained bilateral discussions with officials from the devolved administrations, reporting back to ministers at regular intervals to ensure sufficient progress is being made".
"There is also a place for multilateral meetings and we will take that forward as and when it is appropriate," he told Plaid Cymru MP Jonathan Edwards in a written answer to a question on when the next meeting would be held.
A Welsh Government spokesman said First Minister Carwyn Jones and the Cabinet Secretary for Finance and Local Government Mark Drakeford had repeatedly asked the UK government to restart the meetings each month.
The Scottish government's Brexit minister Michael Russell said the work of the committee was "vital" in ensuring the UK's devolved administrations are properly involved in the Brexit negotiations.
"It is essential, and increasingly urgent, that a next meeting of the JMC is arranged now so that it can do the job it is supposed to do - provide oversight of negotiations with the EU - and we have been working closely with the Welsh Government to achieve such a meeting," he said.
A UK government spokeswoman said it was committed to working with the devolved administrations "as we deliver a successful Brexit for the whole of the UK".
"Since the election, ministers and officials have continued to be in close contact with the devolved administrations," she said.
She added that there were plans for a meeting in early September between the Welsh Government, the first Secretary of State Damian Green and Welsh Secretary Alun Cairns "demonstrating the UK government's commitment to engaging with the devolved administrations".