Labour MP Jacinda Ardern said she believed Red Peak should be in the referendum and denied Labour was blocking the way by adding conditions to its support.
She said Prime Minister John Key could subsitute Red Peak for another shortlisted design, which would not require a law change.
However, she would not say if Labour would support that or criticise Key if he did move to substitute it.
Act leader David Seymour took a swipe at both Mr Key's refusal to substitute it and Labour's position. He said their demand to change the order of the referendum questions had "overly complicated" matters.
"That's made it very difficult for everybody involved. The important thing is ultimately in a democracy, what do the people think. What we've seen today is that 50,000 people got behind Red Peak in a couple of weeks. That's probably the largest amount of spontaneous enthusiasm we've seen throughout this entire process."
Mr Seymour wears a silver fern lapel pin but that did not mean he favoured a silver fern flag.
"You don't have to have every national logo on your flag. The French love the coq, the English have the rose - that's not on their flag."
He said the Prime Minister should withdraw one of the least popular and Red Peak take its place - something Key has ruled out, saying he would be criticised for it.
Mr Seymour said both Kyle Lockwood designs were similar and one could be removed without causing too much upset. He also pointed at the Koru design as having little traction in public opinion.
Prime Minister John Key said he would add it as a fifth option if Labour agreed and threw its support behind the flag process rather than criticise it.
In return, Labour leader Andrew Little said he would give Labour's support but only if the Prime Minister included the question on whether the flag should change in the first of the two planned referendum - something that would require a law change.
Mr Key ruled that out, saying the order of the questions was decided after advice from officials that it would bias the referendum in favour of the current flag if there was no alternative to decide against.