Theresa May has made David Davis's job more difficult by setting "red lines" for him in Brexit talks, his ex-chief of staff has told the BBC.
James Chapman said the Brexit secretary had been "hamstrung" by the prime minister's stance on the European Court of Justice (ECJ), among other things.
He said Mrs May would not get a Brexit deal through Parliament unless she showed more "flexibility".
Downing Street and the Department for Exiting the EU declined to comment.
Mrs May has insisted the ECJ will have no jurisdiction over the UK.
But the EU insists that the ECJ must continue to offer legal protection for its citizens in the UK, just as it does now.
The ECJ's main role is to uphold the rules of the single market, rather than rule on criminal matters like the European Court of Human Rights.
Mr Davis has said a new international body will have to be set up to settle disputes between the UK and the EU after Brexit, a job currently done by the ECJ.
Mr Chapman accused Mrs May of taking an "absolutist" position on the ECJ, saying: "She's set a red line effectively for a conference speech that hamstrung these negotiations in my view."
He added: "There have been red lines that have been set for him that make the job he has to do very difficult."
Mr Chapman also warned, in an interview with The Week in Westminster to be broadcast on Saturday at 11:00 BST on BBC Radio 4, that Mrs May would struggle to get her version of Brexit past MPs.
"If she doesn't, in my point of view, show more flexibility, show more pragmatism than she did demonstrate in the Home Office, she won't get this stuff through Parliament."
A former Daily Mail journalist, Mr Chapman was George Osborne's director of communications before becoming Mr Davis's chief of staff at the new department for exiting the EU.
He also revealed that cabinet ministers wanted Mrs May to do a U-turn over plans to pull the UK out of Euratom, the pan-European atomic energy regulator.
Euratom is a separate legal entity from the EU and gives Britain's nuclear industry access to technology and fissile material.
Mr Chapman said the reason for wanting to withdraw from Euratom was to prevent the free movement of nuclear scientists, which is governed by the ECJ.
"Now I would have thought the UK would like to continue welcoming nuclear scientists, who are all probably being paid six figures and are paying lots of tax," he said.
"But we're withdrawing from it because of this absolutist position on the European Court."
He added: "If she doesn't shift on this I think Parliament will do it for her."
He also took a swipe at the ability of ministers in Mrs May's top team, which he said was "not groaning with talent".
"I think a political party is in a bad place when there's more talented people on its back benches than there are on the front benches."
He said the cabinet's leading "Brexiteers", David Davis and Boris Johnson, were "actually pretty liberal on issues like immigration" and would like to "recalibrate" Mrs May's position, "but at the moment she is showing no willingness to do this".
Mr Chapman stopped working for David Davis at the election and is now a partner at lobbying company Bell Pottinger.