It would be “unthinkable” if a no-deal Brexit was not followed by a poll on Irish reunification, the leader of Sinn Féin has warned Boris Johnson, also telling the prime minister that no one believed he was impartial on Northern Ireland.

“In the longer term, we have advised him that constitutional change is in the air. He can’t say that he hasn’t been told,” Mary Lou McDonald said after meeting Johnson at Stormont on Wednesday morning.

Any Brexit, but particularly no deal, “represents in anybody’s language a dramatic change of circumstances on this island, and … it would be unthinkable in those circumstances that people would not to be given the opportunity to decide on our future together”, McDonald said.

Johnson held bilateral meetings with the five main parties at Stormont on Wednesday, ostensibly about deadlock in the latest talks process. Northern Ireland’s devolved assembly and executive have been suspended since January 2017.

After the meetings the DUP leader, Arlene Foster, said Johnson had dismissed the idea of a reunification vote, adding: “Talk of a border poll, he told us, was not something that he was entertaining.” The PM “reiterated the fact that he would never be neutral on the union”, she said.

Nichola Mallon, deputy leader of the more moderate nationalist SDLP, said Johnson delivered only “bluff and bluster around Brexit”.

She said: “We went into this meeting concerned that he would have a limited understanding of the complexities and the fragility of this place and those concerns have been confirmed. It is very clear he views and understands the situation through the eyes of the DUP.”

A Downing Street statement said Johnson “told all of the parties that he was determined to bring this process to a successful conclusion and that he would do everything he could to make it happen”.

The No 10 statement said that in all his meetings Johnson “made clear that the UK would be leaving the EU on 31 October come what may, and restated his intention to do so with a deal.

“He said that in all scenarios, the government is steadfast in its commitment to the Belfast-Good Friday agreement and that in no circumstances would there be physical checks or infrastructure on the border.

“He also made clear his belief and commitment in the rigorous impartiality set out in the Belfast-Good Friday agreement, while at the same time reaffirming his determination to strengthen the union and Northern Ireland’s place within it.”

However, McDonald said she was sceptical about Johnson’s supposed neutral role. “He tells us that he will act with absolute impartiality. We’ve told him that nobody believes that. Nobody believes that because there are no grounds to believe that there is any kind of impartiality, much less strict impartiality.”

She said Johnson needed to stop being “the DUP’s gofer”. “We’ve made clear to him that the ongoing indulgence of the DUP and rejectionist unionism needs to stop.”

Speaking earlier on BBC Radio 4’s Today programme, the DUP chief whip, Sir Jeffrey Donaldson, said he believed there was a significant chance of a no-deal Brexit but that the outcome would be down to Irish and EU intransigence. He also played down the prospect of a border poll.

“I think given the response of the Irish government in particular, who I believe are key to this issue of addressing UK concerns about the backstop, I think the prospect of a no deal is significant,” he said.

Donaldson said warnings of 40,000 job losses in Northern Ireland were at the “very high end of the scale”.

﻿Johnson spent Tuesday night dining with Donaldson, as well as the DUP leader, Arlene Foster, and her deputy leader, Nigel Dodds, which No 10 sources said was a discussion about the DUP’s confidence and supply agreement with the Conservatives, not Northern Ireland power-sharing talks. The Northern Ireland secretary, Julian Smith, was not present.

Johnson talked to the Irish taoiseach, Leo Varadkar, on Tuesday afternoon, the first time the two leaders had spoken since he became prime minister, during which the pair clashed over the reopening of the withdrawal agreement.