In his resignation statement to Britain's House of Commons, former foreign secretary Boris Johnson has said the UK has "allowed the question of the Northern Irish border to become so politically charged as to dominate the debate".
Mr Johnson resigned last week after disagreeing with Prime Minister Theresa May over her Brexit plan agreed at Chequers.
Mr Johnson said: "We accepted the jurisdiction of the European Court over key aspects of the withdrawal agreement and worst of all we allowed the question of the Northern Irish border, which had hitherto been assumed on all sides to be readily soluble, to become so politically charged as to dominate the debate."
Mr Johnson insisted that checks away from the Northern Irish border and technical solutions were possible.
He cited concerns raised by himself and former Brexit secretary David Davis, saying: "When I and other colleagues... proposed further technical solutions to make customs and regulatory checks remotely, those proposals were never properly examined, as if such solutions had become intellectually undesirable in the context of the argument."
Mr Johnson said: "We need to take one decision now before all others and that is to believe in this country and what it can do."
Mr Johnson said it became "taboo to even discuss technical fixes" regarding the Irish border.
He said "a fog of self-doubt has descended" over Mrs May's plans for a free trade agreement with the EU after Brexit.
'It is not too late to save Brexit', Johnson says in resignation speech | https://t.co/JWuVrlcxyn pic.twitter.com/7QAYMwztPx
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Mr Johnson told the Commons: "We are volunteering for economic vassalage." He said the government had "dithered" over the Lancaster House strategy agreed last year.
He said: "We never actually turned that vision into a negotiating position in Brussels and we never made it into a negotiating offer.
"Instead we dithered and we burned through our negotiating capital. We agreed to hand over a £40bn exit fee with no discussion of our future economic relationship."
He added: "After 18 months of stealthy retreat we have come from the bright certainties of Lancaster House to the Chequers agreement."
Mr Johnson said that Mrs May's Chequers plan amounted to "Brexit in name only" and he was unable to accept it or support it.
"I'm happy now to be able to speak out against it," he said.
"It is not too late to save Brexit. We have time in these negotiations. We have changed tack once and we can change again.
"The problem is not that we have failed to make the case for a free trade agreement of the kind spelt out at Lancaster House. We haven't even tried.
"We must try now because we will not get another chance to do it right."
Earlier, Mrs May said the UK needed to make clear to the European Union that it was preparing for a no-deal outcome to the Brexit negotiations, despite wanting to leave with an agreement.
"We do need to make sure that we have those no-deal preparations in place while we negotiate with the European Union on a deal," she said.
"The European Union need to be in no doubt that we are making those preparations," she added.
Meanwhile it was reported that Mrs May threatened rebels in her Conservative Party with a general election this summer if they defeated her Brexit plans on customs, The Times newspaper reported.
The newspaper reported that Conservative whips, who enforce discipline in the party, issued the warning to pro-EU MPs, led by the former ministers Stephen Hammond and Nicky Morgan, minutes before a crucial vote on customs last night.
Mrs May narrowly avoided a defeat in parliament at the hands of the pro-EU members from her own party in the vote.
Meanwhile, Theresa May has met a group of her backbenchers in Westminster tonight following a day of Committees and questions on her Brexit strategy.
One MP at this evening's meeting is said to have told Mrs May that he had put in a letter calling for a leadership contest, but was now withdrawing it.
Parliament voted 307 to 301 against an amendment to trade legislation that would have required the government to try to negotiate a customs union arrangement with the EU if, by 21 January 2019, it had failed to negotiate a frictionless free trade deal with the EU.
In one of the most tumultuous periods in recent British political history, there have been four major elections in the past four years: the Scottish independence referendum of 2014, the 2015 UK general election, the Brexit referendum of 2016 and the snap general election called by Mrs May last year.
