David Cameron has been accused by a pro-Brexit former cabinet colleague of allowing Germany to "veto" key parts of his EU reform renegotiation.
Iain Duncan Smith said Berlin exercised the "ultimate power" over what changes the Prime Minister sought from Brussels and was allowed to block the idea of a cap on foreign workers coming to the EU.
The ex-Tory leader, who quit as work and pensions secretary in March in protest at disability benefit cuts, accused Mr Cameron of being "compliant" as Conservative divisions on Europe continued to rage.
He will on Tuesday follow Justice Secretary Michael Gove and former London mayor Boris Johnson is making high-profile speeches on the case for a "leave" vote in the referendum on June 23.
In an interview with The Sun, he said a key demand was ditched, at the behest of Berlin, from the draft of a key speech by Mr Cameron just hours before it was due to be delivered.
"It's like they were sitting in a room, even when they were not there. There was a spare chair for them - called the German chair. They have had a de facto veto over everything," he told the newspaper.
"I know that right up until the midnight hour, there was a strong line in there about restricting the flow of migrants from the European Union - an emergency brake on overall migration.
"That was dropped, literally the night before. And it was dropped because the Germans said if that is in the speech, we will have to attack it."
He went on: "There is no question in my mind that keeping the Germans on side was the only thing that really mattered.
"We wanted to use the Germans to work the others in the room. They had the ultimate power over it."
The PM's failure to announce a cap or "emergency brake" on the overall numbers coming to the UK in the November 2014 speech setting out his demands - despite media reports he would do so - disappointed Eurosceptic Tory MPs,
Foreign Secretary Philip Hammond denied at the time that Mr Cameron "backed off" in the face of pressure from German chancellor Angela Merkel and said simply that the UK had "sought to work with our partners in the European Union".
In the eventual deal, the UK was granted to right to apply for a seven-year "emergency brake" under which new migrants will only receive the right to claim in-work benefits gradually over the course of four years.
"We put ourselves in a compliant position to another country which doesn't have your best interests necessarily at heart," Mr Duncan Smith said.
"We are now in a worse position than we were before. We have gone from wanting to lead in Europe to being on the end of a lead in Europe."
A Number 10 source told The Sun: "The Prime Minister made clear at the time that the Government had looked at an emergency brake but he decided it was not the most effective way forward.
"That is why he decided to impose restrictions on benefits instead to end the something for nothing culture."
Mr Duncan Smith said the Remain camp had "thrown the kitchen sink at us" to little avail as opinion polls had not shifted in favour of staying in the EU.
He said he was not "in any way wanting to go back into government" after the vote.
"I didn't leave for the European bit. I left because of my concern that the direction of travel on social reform".