David Cameron's hopes of renegotiating Britain's relationship with the EU have received a boost after Germany said the UK must not be put at a disadvantage by eurozone nations' moves to integrate more closely.
In a joint article for the Financial Times, Chancellor George Osborne and his German counterpart Wolfgang Schauble said any changes to EU treaties must "guarantee fairness" for member states outside the single currency bloc.
The declaration meets one of Cameron's key negotiating demands - that the rights of non-eurozone states must be protected - ahead of his planned 2017 referendum on Britain's membership of the EU.
In their article, Osborne and Mr Schauble said that both Britain and Germany accepted that the eurozone crisis meant that the members of the single currency needed to establish a common fiscal and economic policy.
"The UK fully recognises the progress made so far in responding to the crisis, and it supports the case for further steps forward. But as the euro area continues to integrate, it is important that countries outside the euro area are not at a systematic disadvantage in the EU," they wrote.
"So future EU reform and treaty change must include reform of the governance framework to put euro area integration on a sound legal basis, and guarantee fairness for those EU countries inside the single market but outside the single currency."
The statement will be seen as evidence that Cameron's assiduous efforts to court German chancellor Angela Merkel - including rolling out the red carpet for a high-profile visit to London - are bearing fruit.
Earlier he indicated that they intended to work closely together on measures to enable the expulsion of EU migrants who fail to find work.
Germany's coalition government is considering proposals to limit the time unemployed migrants from other EU member states can stay in the country looking for a job. An official report also recommended that welfare abuses by EU migrants should be punished by a temporary entry ban.
Cameron said that he would now seek to work with the Germans and other like minded allies to see if they could tighten EU rules further.
"We'll look very closely if the Germans want to take this further," he told BBC Radio Derby.
"I talked to Angela Merkel personally last week and we are going to work with her, with the Swedish prime minister, with the Dutch prime minister, on how we can tighten up in this area because it is worthwhile work."
The Prime Minister has been under intense pressure to strengthen EU immigration rules from Tory MPs worried about the growing strength of the UK Independence Party.
In his broadcast debate with Deputy Prime Minister Nick Clegg, Ukip leader Nigel Farage repeatedly raised the issue, warning 485 million people across the EU - many from very poor countries - were currently entitled to settle "unconditionally" in the UK.