The prime minister's Brexit negotiating plan has left her with "no room to manoeuvre", the DUP's Sir Jeffrey Donaldson has said.
Sir Jeffrey said his party was prepared to give Theresa May "some leeway to try and negotiate the best deal".
On Friday the government published its blueprint for UK relations with the EU.
Mrs May - who relies on the Democratic Unionist Party (DUP) to win key votes - has urged her critics to support her plans.
The long-awaited White Paper is aimed at ensuring trade co-operation, with no hard border for Northern Ireland, and global trade deals for the UK.
Speaking on the BBC's Sunday Politics programme, Sir Jeffrey said his party had studied the White Paper over the weekend and would meet the government this week to "put forward any concerns".
"We are entitled to put forward our views," he said.
"The White Paper is a negotiating position - not the final outcome. It's important for us that our red lines have clarity."
No deal was better than a bad deal, he added.
Tory Brexiteer Jacob Rees-Mogg called it a "bad deal for Britain".
Mr Donaldson said his amendment to effectively kill off any idea of an Irish Sea border was "sensible" and he hoped the government would adopt it.
"It's important to have it in the legislation - it's entirely consistent with what the prime minister has said," he said.
The White Paper, which fleshes out the Chequers agreement that sparked the resignations of Boris Johnson and David Davis, was welcomed by the DUP leader Arlene Foster.
"As far as we are concerned, there has always been the one red line in relation to borders between ourselves and the United Kingdom and we are content that that has been reflected at the Chequers summit and also by the White Paper," she said.
Key dates at-a-glance
18 October 2018: The key EU summit. Both sides hope to agree outline of future relations to allow time for UK parliament and EU members to ratify deal by Brexit day
13 December 2018: EU summit. If deal not done by October, this is the fall back option if the two sides still want to reach agreement
Commons and Lords vote on withdrawal treaty - MPs could reject the deal but it's not clear what would happen if that is the case
The UK Parliament also needs to pass an implementation bill before Brexit day
29 March 2019: As things stand, deal or no deal, Brexit is due to happen at 11pm UK time
31 December 2020: If all goes to plan a transition period will then last until midnight on this date