Germany backs UK over calls for 'fairness' in EU reforms
David Cameron's hopes of renegotiating the UK's place in the European Union have been boosted after Germany said any future changes must be fair to nations not using the single currency.
Its finance minister Wolfgang Schauble said countries outside the eurozone should not be put at a "systematic disadvantage" by future integration.
Writing in the Financial Times, he said legal protections must be put in place.
No 10 said it amounted to the German government publicly backing its case.
Mr Cameron has said that if the Conservatives win the 2015 election, he will seek to renegotiate the terms of the UK's membership of the European Union and put the outcome to an in-out referendum of the British people in 2017.
The prime minister says moves towards deeper economic and political integration within the eurozone are a trigger for the UK to secure a "better deal" in Europe and redraft the terms of its membership.
He has suggested that existing EU treaties will need to be rewritten - although France has signalled that it does not believe this is a priority at the moment.
The UK has long maintained that safeguarding national interests, such as fair access to the European single market and protecting the City of London, will be vital in any future process.
In the joint FT article, Chancellor George Osborne and his German counterpart said any changes to EU treaties must "guarantee fairness" to all member states.
"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," the two said.
The pair added: "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."
It is the first time Germany has publicly indicated that countries such as the UK must not be put at a disadvantage by eurozone nations' moves to integrate more closely.
Downing Street said Mr Schauble's comments amounted to the German government stating publicly for the first time that safeguards for non-eurozone states should not simply be negotiated on a case-by-case basis as the single currency area takes further steps towards integration, but should be underpinned formally in the EU treaties.
A No 10 spokesman said it was vital that there was a "level-playing field" between countries inside and outside the euro.
"What we see is Germany, one of the leading players in the eurozone, accepting that there needs to be these proper safeguards put in place as further changes are made for the eurozone."
"This is a German finance minister coming out and saying there will be treaty change and it must include reform of the framework to ensure the right safeguards between euro-ins and euro-outs."
The BBC's Ben Wright said the prime minister and Conservative backbenchers would be delighted that the Germans were "clearly receptive" to their concerns.
The timing of the comments would also be welcome in No 10, he added, coming as the debates between Nick Clegg and Nigel Farage have put the issue of what can be achieved in Europe top of the political agenda.
But this would not be enough on its own, he added, to satisfy Conservative MPs who have been promised a return of powers from Brussels to London.
Mr Cameron has been seeking to get Germany's backing for its reform agenda in Europe but on a visit to the UK earlier this month, Chancellor Angela Merkel bwarned it would not be "a piece of cake".
Reacting to Friday's development, Conservative MP Mark Reckless said the dialogue between Berlin and London was "excellent" but the UK had to decide what kind of relationship it wanted with the eurozone.
"Although the Germans are not going to remake the EU to suit the UK, and the prospect of treaty change does seem to be receding, Germany sells more to the UK than any other country in the world and it is clear they will be keen to maintain free trade and friendly relations were we to vote for a future outside the EU," he told Radio 4's World at One.
But Conservative MP Douglas Carswell struck a cautious tone about what was being promised.
"Hurray! British and German governments promising us EU reform and "subsidiarity" in the FT. Just like the 1990s all over again," he tweeted.