His comments come after European Union leaders rejected Theresa May's Chequers plan at their Salzburg summit on Thursday. The Environment Secretary said that while the blueprint was "the right one for now", a future prime minister would have the power to "alter the relationship".
"Hardliners will fight back by saying that the United Kingdom is throwing away its best card - the £39 billion divorce payment to the European Union - for vague warm words about the future relationship and so would have little or no leverage during the talks on it during the transitional phase", said Rahman.
"On the economic partnership, there is no solution that will resolve the Northern Ireland border which is not based on the frictionless movement of goods", she said. Brussels has baulked at May's proposals for a future customs arrangement and for Northern Ireland.
May defended her Chequers proposal for Brexit on Wednesday when arriving in Salzburg by saying that "It's the only credible and negotiable plan on the table that delivers no hard border in Northern Ireland and also delivers on the vote of the British people".
Their Brexit aim for this summit was to go as far as they could with words and gestures to throw Theresa May a lifeline.
It feels that the search for something else has been in vain. There are things that I wouldn't personally accept, but nor would the Prime Minister.
She repeated that she would bring forward alternative proposals to the backstop, which would come into effect until a new trade deal is struck.
While progress was being made in negotiations, he pointed out that it was "not as simple as suggested by those who promised the British people the Earth at the time of the referendum".
May said that leaving with no deal remains an option.
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But Mrs May indicated the United Kingdom will unveil new measures on the future status of the Northern Irish border in a bid to break the deadlock.
"They build up, there's tough language and actually a deal is done at the last. I fully believe that we will be able to find an agreement".
Irish Prime Minister Leo Varadkar said there was no chance the European Union would compromise on its demands and rejected speculation the other countries would be tempted to fudge the Irish issue until later.
But he added negotiations were not yet at an "end game" and said Chequers was "broadly" a package that could get through Parliament.
But she showed no sign of backing down from the proposals hashed out with her Cabinet at her country retreat of Chequers in July - so controversial with her own party that they provoked the resignation of her foreign secretary, Boris Johnson, and Brexit secretary, David Davis.
He told HuffPost UK that, with Labour also planning to vote against Mrs May's plan, the "critical size of the voting bloc is quite small, it's basically a dozen people". It strengthens the hand of the Brexiteers and calls into question her leadership in a way that wasn't the case 48 hours ago."David Davis, the former Brexit secretary, is to publish his alternative plan for a free-trade deal before this month's Tory conference in a move created to further embarrass Mrs May.Nicky Morgan, chairwoman of the Treasury select committee, said: "Chequers has been in difficulty for some time".
Tusk, meanwhile, said that the upcoming October summit would be the "moment of truth" for reaching a deal, and that "if the conditions are there" another summit would be held in November to "formalize" it.
"Europe isn't an a la carte menu", French President Emmanuel Macron said, noting that "as it stands, the Chequers plan seems to be a take-it-or-leave-it plan".