Boris Johnson's call to scrap the Northern Ireland backstop has been called "foolish" and "incendiary".
The MP has hit out at Theresa May's Brexit plans, accusing her of wrapping a suicide vest around the UK.
Sinn Féin said the comments showed Northern Ireland was "collateral damage" for party in-fighting.
The SDLP said Mr Johnson was "deliberately simplifying" the Irish border issue. However, the DUP welcomed the former foreign secretary's input.
The UK and the EU signed up to the principle of a backstop in December 2017, but neither can agree on what it should look like in a legally binding text.
The EU has proposed a backstop that would mean NI staying in the customs union, large parts of the single market and the EU VAT system.
But the UK government has rejected it as a threat to the integrity of the UK and has suggested a backstop that would see the UK as a whole remaining aligned with the EU customs union for a limited time after 2020.
In his piece for the Mail on Sunday, Mr Johnson accused the EU of "bullying" the UK - but questioned why the response had been "so utterly feeble".
He said the reason for this was "simple", namely Northern Ireland and the "insanity of the so-called backstop", and called for it to be scrapped.
Mr Johnson said agreeing to a backstop had "opened ourselves to perpetual political blackmail" and given Brussels the ability to "crack apart the Union between Great Britain and Northern Ireland".
Speaking to BBC News, Sinn Féin MLA Máirtin Ó Muilleoir said Mr Johnson's comments were "appalling" and the language was "foolish and dangerous".
"Anyone who doesn't understand how fragile the peace here is, and how fragile the progress we have made is, mustn't be watching the news because for 600 days we haven't been able to form a local government here because fault lines within this society remain," he said.
The SDLP's Brexit Spokesperson Claire Hanna said Mr Johnson had "absolutely no regard for people on this island".
"He is completely indifferent to the complexities of issues here; he is deliberately simplifying things and is implying the border issue is about marking a line and completely ignoring regulatory issues, as well as all of the political issues that plummet into peace and the Good Friday Agreement."
However, Sammy Wilson from the DUP welcomed the former foreign secretary's input.
In a statement the MP for East Antrim said he wished Mr Johnson had made the comments sooner and that he believes the backstop needs to be rewritten.
"Those crying loudest are embarrassed at the truth of the impact of the arrogant EU demands on the UK being exposed," said Mr Wilson.
"Boris Johnson is saying nothing that the DUP has not been saying since the withdrawal agreement was first drafted in December 2017.
"The reason why we stopped it being signed in its original form was precisely for the reasons which Boris Johnson has outlined."
The UK is due to leave the EU in just over six months' time.
During the summer the EU's chief negotiator, Michel Barnier, referred on several occasions to the EU's willingness to "improve" the backstop.
But he gave no indication that there would be significant movement on its substance.
Several of the pro-remain parties in Northern Ireland are due to travel to Brussels later this month to meet Mr Barnier about the impact of Brexit on Northern Ireland.