A former government Brexit advisor has suggested the DUP made a mistake by accepting there could be regulatory differences between NI and the rest of the UK.
Raoul Ruparel, who advised Theresa May, was giving evidence to a Commons committee on Wednesday.
He said one of the reasons Boris Johnson got a deal with the European Union was due to the DUP's move.
But added "that maybe wasn't the right choice" for the DUP.
The DUP supported Boris Johnson's original offer to the EU which would have meant Northern Ireland diverging from the UK on some regulations, but only with a unionist veto.
However, Mr Johnson later agreed a deal with the EU which will mean Northern Ireland diverging on regulations and customs without a unionist veto.
The DUP have been strongly critical of that deal.
Mr Ruparel said the DUP's decision in September to accept the principle of divergence gave Mr Johnson more "leeway" in his negotiations with the EU.
He said that at the time Theresa May was negotiating with the EU, the DUP were "not willing" to allow the government to move in that direction.
"They had previously been opposed to any sort of regulatory differentiation," he told the committee.
He added that the end result of that "significant change" was "obviously not something the DUP ended up agreeing with".