Berlin has already made perfectly clear that any changes to European treaties would be too risky.
This is not just a breach of the solemn and absolute prohibition of torture to which the US has committed itself in international treaties: it is a terrible political and moral mistake.
The difficulty is that some of these reforms might require changes to European treaties, raising the prospect of Labour needing to support an in/out referendum on Britain's EU membership.
"There are some possibilities to make changes in the regime for benefits for people from other parts of the union, but without change in the treaties it is very difficult to make any important change," he told the BBC's Andrew Marr Show.
Germany has made it clear that it will not rush to change EU treaties, and eastern European ministers have spoken out against curbs on proposed migrant workers' rights, which discriminate against their citizens.
The bigger issue for many is the lack of clarity or consistency in the language of treaties, and the role of precedents.
"Renegotiating treaties is definitely not an option," Gunther Krichbaum, the CDU chairman of the commission on European affairs, told the Guardian.
While banking union and further economic integration in the eurozone will require some changes to the treaties, the German government would prefer to do so without having to reopen old agreements entirely: Vertragsanpassungen, "treaty modifications" rather than "treaty change", is the phrase her party uses.
The fact is that this review was open to anybody to contribute, yet the overwhelming evidence received strongly supported the UK remaining in the EU and that an attempt to radically rewrite the treaties is neither achievable nor desirable".
On Sunday, José Manuel Barroso, the former European commission president, suggested Cameron's proposed limits to benefits may be possible, though "very difficult" without reopening EU treaties.
They insist there can be no question of opening up European treaties for reform, as Cameron demands, nor of any fundamental change to EU migration rules – Ukip's main hobby horse.
