Tony Blair has urged European leaders to reform the EU so British people "change their mind" about Brexit.
The former prime minister argued that if "comprehensive" immigration reforms are offered, voters will realise their "genuine underlying grievances" can be addressed.
He thinks Brexit can then be "averted" via another referendum, this time on the final deal reached with the EU.
Mr Blair, who opposed Brexit, also said Northern Ireland could be at risk.
Speaking to BBC Radio 4's Today programme, he said: "I find it not just disappointing but sickening that people should really be prepared to sacrifice peace in Northern Ireland on the altar of Brexit."
Brexit campaigners dismissed his remarks.
"Former prime ministers who no longer believe in our great nation are engaged in a desperate last ditch attempt to defy the largest democratic mandate ever in the UK," said Leave Means Leave's Richard Tice.
"Disgracefully, they are using the Irish border as their proxy, ignoring the technological solutions that solve the issue."
Mr Blair's speech, in Brussels, was the second warning about leaving the EU by a former prime minister in two days, following Sir John Major's intervention on Wednesday.
'Hammer the Tories'
He said that Labour should "say what it really believes" on Brexit - which was that leaving the EU will "make problems worse".
If he was still in charge, he said, "I would be hammering the Tories all the time" on the "destructive impact" of Brexit.
"We could be making that case so forcefully," he added.
Mr Blair earlier set out the three steps he said could lead to a "reconsideration of Brexit".
These are firstly showing voters that Brexit "has turned out much more complex and costly than they had thought", secondly, responding to their grievances, especially around immigration, and thirdly the EU accepting the vote as a "wake-up call" to change.
"Reform in Europe is key to getting Britain to change its mind," he told the European Policy Centre think tank, calling for "a comprehensive plan on immigration control, which preserves Europe's values but is consistent with the concerns of its people and includes sensitivity to the challenges of the freedom of movement principle".
He also wants a "roadmap for future European reform".
Conservative MP Nigel Huddleston hit back, tweeting: "As Tony Blair lectures today's politicians on what we should be doing on Brexit one wonders if he has the self-awareness to realise one of the key reasons we are leaving the EU is because of his inability to control immigration when he was PM for a decade."
Mr Blair's views on Brexit are not shared by the present-day leadership of the Labour Party - Jeremy Corbyn says the result should be respected and is not calling for a second referendum.
Speaking on the Today programme, Mr Blair said Mr Corbyn's recent commitment to a customs union with the EU was "sensible", but warned Labour will "very soon find that we've got to move further in order to escape the dilemma ourselves".
On Friday Prime Minister Theresa May will deliver a major speech setting out the UK's strategy for the next phase of the negotiations, including striking a new free trade deal with Brussels.
On Wednesday she attacked EU proposals for Northern Ireland to stay in the customs union, saying they undermined the UK's constitutional integrity.
Foreign Secretary Boris Johnson has suggested the row about Northern Ireland is being stoked by people who want to "frustrate" Brexit.