Tony Blair has hit back at criticism from the prime minister, warning her that it would be irresponsible to try and "steamroller" her Brexit agreement through.
The former Labour prime minister and Theresa May are embroiled in a row after she accused him of "undermining" the EU divorce talks by calling for a second referendum, which she said was an "insult to the office he once held".
But Mr Blair has dismissed the charge and said he was acting in the national interest.
The spat came as two of the prime minister's closest aides were forced to deny claims they were planning for a fresh Brexit poll.
Labour has also vowed to do all it could to try and force a Commons vote this week on the PM's withdrawal agreement, which would almost certainly be defeated.
Responding to Mrs May's criticism, Mr Blair said: "To describe such a course as an insult is a strange description of what would be the opportunity for them to instruct Parliament as to how to proceed.
"Far from being anti-democratic, it would be the opposite - as indeed many senior figures in her party from past and present have been saying.
"What is irresponsible however is to try to steamroller MPs into accepting a deal they genuinely think is a bad one with the threat that if they do not fall into line, the government will have the country crash out without a deal."
The former Labour leader said he was within his rights to offer advice when the country was "in crisis" over "the most important decision our country has taken since the end of World War Two".
Mr Blair said it was "perfectly clear neither the British people nor their parliament will unite behind the prime minister's deal" and that was why she was forced to pull the vote last week.
"In these circumstances it is not irresponsible or insulting to put forward an alternative way to achieve resolution," he said.
"The sensible thing is now to allow parliament to vote on each of the forms of Brexit canvassed including the prime minister's deal.
"If they can't reach agreement then the logical thing is to go back to the people."
But International Trade Secretary Liam Fox ruled this out, arguing a second referendum would "perpetuate" the deep divisions in Britain.
However, he indicated he could support a free vote for MPs on Brexit options.
Dr Fox told BBC1's The Andrew Marr Show: "That's not something we have considered.
"I have to say, personally, I wouldn't have a huge problem with parliament as a whole having a say on what the options were."
Education Secretary Damian Hinds also insisted the cabinet had not discussed a second EU referendum.
Asked if the cabinet had talked about the issue, Mr Hinds told Sky News' Sophy Ridge on Sunday programme: "No. Government policy couldn't be clearer. We are here to act on the will of the British people clearly expressed in the referendum."
Warning that another vote would be "divisive", Mr Hinds added: "We've had the people's vote, we've had the referendum and now we've got to get on with implementing it."
His comments came as the PM's defacto deputy, Cabinet Office minister David Lidington, and Mrs May's chief of staff Gavin Barwell, denied reports they were involved in planning for a new Brexit referendum.
Mr Barwell tweeted: "Happy to confirm I am *not* planning a 2nd referendum with political opponents (or anyone else to anticipate the next question)"
One of those arguing for a second referendum is former Tory minister Jo Johnson, who quit the government in protest over Brexit.
He told Sky News: "My priority is to ensure that we don't take an irrevocable step by accepting this bad deal. It's not in our national interest and I think it's really important we think again before we do that."
Meanwhile, Labour has said it will not bring a confidence motion against the government until after a meaningful vote on Mrs May's Brexit deal is held.
Shadow communities secretary Andrew Gwynne, who is also the party's election co-ordinator, told the BBC: "Parliament has to decide whether or not it supports the prime minister's deal and fundamentally until we secure that meaningful vote from the parliament we can't move to the next stage.
He added: "We will be using whatever mechanisms we have at our disposal next week to try and force the government to bring forward that deal for a vote before Christmas."
But Scotland's First Minister Nicola Sturgeon said she believed a confidence motion against the government could succeed and criticsed Labour's inaction.
She told Sky News: "Labour's position right now is it won't back a second EU referendum until it has tried and failed to trigger a general election, but if it won't try to trigger a general election then we're in this catch-22 position.
"It seems to me right now that Labour is as much of a barrier to making progress on Brexit as the Tories are."