A senior Democratic Unionist party MP has insisted the party can only support Theresa May’s revised Brexit deal if the withdrawal agreement itself is amended to make the Northern Ireland backstop time-limited, or allow the UK to withdraw unilaterally.

Sammy Wilson, the DUP’s Brexit spokesman, also dismissed worries about the impact on the region of a no-deal Brexit, saying warnings about this on Tuesday from the head of Northern Ireland’s civil service were “politically motivated”.

With May’s revised plan due to be voted on by MPs next week, and little sign her attorney general, Geoffrey Cox, has managed to persuade the EU to shift its position on the backstop during talks in Brussels, Wilson’s continued hard line will concern Downing Street.

Appearing before the Northern Ireland select committee, Wilson was asked by Sylvia Hermon, the independent unionist MP, whether the DUP could agree to so-called enhanced arbitration to limit the backstop, the insurance policy intended to prevent a hard Irish border if no permanent deal can be agreed on this in time.

Reports have suggested that Cox, unable to secure a time limit or unilateral exit mechanism for the backstop, has shifted his focus to a beefed up system that would allow an independent body to decide on how the system would operate.

“That certainly is not a mechanism that we would accept,” Wilson said. “That leaves us in exactly the same position as the current withdrawal agreement does, where somebody else decides whether the United Kingdom can break out of the backstop or whether it stays in the backstop.”

Any exit from the backstop had to be “a decision of this parliament, and this government, not some independent panel of judges”, he said.

Asked by the DUP MP Ian Paisley if May remained committed to securing legally binding changes to the backstop, Wilson said: “I hope that she means the same as we do, namely that the agreement will be changed so that it is not possible for the United Kingdom to be held inside the EU after we have decided to leave.”

He added: “It would have to be changes of treaty, because the treaty itself is legally binding. And if you’re going to overcome the legally binding terms of the treaty at present, then you’ve got to have a change in the treaty.

“We have made it very clear that there has to be an ability for a sovereign parliament and a sovereign government to make a decision about the future status of the United Kingdom as a whole in relation to the EU, or part of the United Kingdom in relation to the EU.”

Asked by Hermon if the DUP’s intransigence risked no deal, Wilson said: “If we finish up with no deal it is a result of the intransigence of the EU.”

She then asked him about comments by David Sterling, the head of the civil service in Northern Ireland, to local political parties about potentially grave consequences for the economy and society in the region if no deal took place.

Sterling warned that EU tariffs could kill off some farming and food processing interests. “In effect, there is currently no mitigation available for the severe consequences of a no-deal outcome,” the letter said.

Security could also be undermined, he added: “The planning assumptions include the possibility that, in some scenarios, a no-deal exit could result in additional challenges for the police if the approach appeared to be unfair or unreasonable for some of those most affected.”

Wilson dismissed the warnings. “First of all you’ve got to look at the timing of this letter,” he said. “I know that civil servants shouldn’t get involved politically, but I have no doubt that this was written for a political motive, because he knew what was coming.”

Pushed on Sterling’s comments, Wilson added: “I don’t care if he’s head of the civil service or Santa Claus, he’s got it wrong.”