Former UUP leader Lord Empey has questioned the DUP's support for Boris Johnson's new Brexit proposals.
The veteran unionist said that he did not understand how any unionist could support the "ghastly" deal to leave the European Union.
The Prime Minister's plans would see Northern Ireland apply EU rules on goods, but stay in a customs territory with the UK.
This would create a regulatory barrier for goods crossing the Irish Sea and create a customs border between Northern Ireland and the Republic of Ireland, but Mr Johnson has insisted there would be no need for checks or infrastructure at the frontier.
Speaking to the Belfast Telegraph on Saturday DUP leader Arlene Foster said that a grand committee of local MPs could decide on Northern Ireland's post-Brexit trading arrangements if the Stormont institutions remain suspended.
The Prime Minister's plan gives Stormont the power to decide whether Northern Ireland remains aligned with EU regulations post-Brexit. But the 17-page document does not detail what happens if devolution is suspended.
The DUP had previously said that it could not support Northern Ireland remaining in the EU single market or a border in the Irish Sea.
Mr Johnson's new Brexit proposals have been rejected by the vast majority of Northern Ireland's political parties and business organisations.
Lord Empey said that one of the key arguments for the UK leaving the EU was the chance to negotiate new trade deals.
"Such deals, it was claimed, would give us a chance to grow and become, as some said, the Singapore of the west," the unionist peer said.
“Under her plan, Britain will be able to do such deals, whether with President Trump or whoever, but Northern Ireland will be excluded as we will remain in and governed by the rules of the EU single market and ruled by the EU Courts, another red line that the DUP has broken.
“I believe that those in Northern Ireland who voted to leave the EU never imagined that goods coming from Scotland, England and Wales would have to be checked before they came into Northern Ireland; they never foresaw and were promised no border in the Irish Sea."
The UUP Chairman said that under the proposals traders from Northern Ireland were being asked to treat goods coming from Great Britain as if they were coming from a "third country".
"It’s a ghastly deal, and I am still unable to understand how any unionist could be advocating such measures," Lord Empey said.
“The so called 'democratic consent' proposal, that the Assembly would have to agree every four years to these proposals, will be watered down to consultation. But worse, the long term effects of these proposals will be to align the Northern Ireland economy to the Dublin economy and we will diverge over time from the British economy. Where, from a unionist perspective, does that lead us?”
In an interview with the Belfast Telegraph Mrs Foster said those criticisng her party's support for Mr Johnson's proposals were "searching for relevance".
"They always have to find a point of difference from the DUP. We were lambasted for the prospect of no-deal," she said.
"Then when we work with the Prime Minister to find a deal, the Ulster Unionist Party in particular can't come out quickly enough to condemn it."
Talks between the UK and the EU over the Prime Minister's new proposals had been set to continue on Saturday, however they will not go ahead after the European Commission said EU member states had agreed the proposals “do not provide a basis for concluding an agreement”.
A spokesman said discussions between the two sides would not take place this weekend and instead the UK would be given “another opportunity to present its proposals in detail” on Monday.
The DUP has been contacted in relation to Lord Empey's comments.