There has been "breathtaking ignorance" from "those who believe themselves to be unionists" who oppose the Northern Ireland backstop in the Brexit deal, Sir John Major has said.
The former prime minister made the remark as Tuesday's vote in the Commons was called off.
Theresa May said she would seek assurances from Brussels for the DUP and Brexiteers on the backstop.
It is the insurance policy to avoid a return to a hard border after Brexit.
Mrs May made a statement to MPs in Parliament on Monday afternoon, and said she had listened to "widespread and deep concerns" from MPs about the backstop.
The DUP and Brexiteer MPs have vowed to reject the deal unless the backstop is ditched, because they say any differences for Northern Ireland could threaten the union and damage the economy.
That is because if the backstop took effect, Northern Ireland alone would align with the EU single market in some areas, meaning new regulatory barriers between GB and NI - Labour has described this as a "de facto Irish Sea border".
It has also said it will vote down the deal in Parliament, along with other opposition parties.
Speaking at the inaugural Albert Reynolds Memorial lecture in County Longford, Sir John was critical of those he described as "believing themselves to be unionists".
He warned of the dangers of violence returning in Northern Ireland, if physical checks or infrastructure were put in place at the border again after Brexit.
"Some opinion has shown a breathtaking ignorance of the likely impact unsettling the Good Friday Agreement will have on Ireland, north and south," said the former prime minister.
"To them, the Irish demand for a backstop is a bogus ploy, a bogus ploy to keep the UK in a customs union.
"In truth, a backstop is of vital national interest for Ireland and for the United Kingdom."
He added, however, that despite the turmoil at Westminster, he did not believe that a majority of MPs would permit a hard border to become a reality.
"The reckless few, who are careless of its likely effect are a clear minority, and with good reason."
What has the Irish government said?
Taoiseach (Irish Prime Minister) Leo Varadkar said it is not possible to renegotiate the Irish border backstop proposal without "opening up all aspects" of the Brexit withdrawal agreement.
He stressed the current plan was "the only deal on the table", which took a year-and-a-half to negotiate.
The Irish government has "already offered a lot of concessions along the way", which have the support of the other 26 EU member states, added Mr Varadkar.
The taoiseach confirmed that he had spoken to European Council President Donald Tusk by phone on Monday afternoon and discussed this week's European Council meeting and the current Brexit situation.
Why is Northern Ireland important in all of this?
Both the EU and UK have committed to ensuring the border between Northern Ireland and the Republic of Ireland remains as frictionless after Brexit as it is now.
The government's deal contains the Northern Ireland protocol: the insurance policy to avoid the return of a hard border if a solution cannot be reached through a wider trade deal.
Also known as the backstop, it sparked a political backlash amid claims it could lead to the break up of the United Kingdom.
I can’t follow anymore. After two years of negotiations, the Tory government wants to delay the vote. Just keep in mind that we will never let the Irish down. This delay will further aggravate the uncertainty for people & businesses. It’s time they make up their mind! #brexit— Guy Verhofstadt (@guyverhofstadt) December 10, 2018
The UK would also not be able to leave the backstop without EU agreement.
Sinn Féin, the SDLP, the Alliance Party and the Greens, as well as a number of business and agri-food groups, have urged Parliament to back the deal.
In a statement, the leaders of those parties urged the backstop to be "banked", in order to protect the Good Friday Agreement, they said.
What has the DUP said?
DUP deputy leader Nigel Dodds described the delay on the Brexit vote as a "shambles".
"Please listen and amend the withdrawal agreement or it will be voted down," he warned Mrs May in the Commons.
Just finished a call with the Prime Minister. My message was clear. The backstop must go. Too much time has been wasted. Need a better deal. Disappointed it has taken so long for Prime Minister to listen.— Arlene Foster (@DUPleader) December 10, 2018
The government needed to realise it was "in trouble" when it had crossed the DUP's red line relating to Northern Ireland, he added.
Party leader Arlene Foster said unless changes were made to the withdrawal agreement, the deal "will not fly with the DUP".
The DUP - which props up the government in a confidence-and-supply pact to give Mrs May a working majority - has been putting pressure on the government to "bin the backstop".
Its MPs have already flexed their political muscle by voting against the government several times, in a bid to force the prime minister to change course.
What happens now?
I have decided to call #EUCO on #Brexit (Art. 50) on Thursday. We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification. As time is running out, we will also discuss our preparedness for a no-deal scenario.— Donald Tusk (@eucopresident) December 10, 2018
A summit of EU leaders is still due to take place on Thursday, but the EU Council President Donald Tusk has said Brussels will not renegotiate the deal, including the backstop.
Mrs May has refused to say when the Commons vote on her deal would now be held - saying it would depend how long fresh talks with the EU last.
Some MPs called for it to come back to the Commons before Christmas, but Mrs May would only say the final deadline for the vote was 21 January.