Angela Merkel says Britain will remain an important member of the European Union, despite its refusal to join a new treaty designed to help the EU overcome the eurozone debt crisis.

Merkel, the leader of Europe's biggest economy, told the German parliament that there was no quick fix to the 17-nation currency bloc's crisis, but said it offered an opportunity to achieve an ever more integrated union.

"The vision of a genuine political union is beginning to take shape," the chancellor said, praising the outcome of last week's EU summit to create a new treaty that moves the bloc towards tough debt ceilings and greater fiscal integration.

"The opportunities lying in this crisis are many times greater than its risks, that is my conviction," she said. The new sense of shared responsibility and destiny across the eurozone and the EU's other 10 members would "far outlast this crisis", she added.

David Cameron blocked Merkel's push to bring all 27 EU members closer together for treaty changes last week in Brussels, leaving Britain isolated and forcing others – perhaps all 26 remaining members – to forge a new pact outside the official EU treaties.

Merkel said that, while she regrets the decision, "it is beyond doubt for me that Great Britain will in future continue to be an important partner in the European Union".

"[Britain] is a reliable partner for Europe not just in questions of foreign and security policy … [it] is also this partner in many other questions – in competitiveness, in the internal market, for trade, for climate protection," she said.

Cameron's move drew criticism from within the coalition government, with the deputy prime minister, Nick Clegg, calling the decision bad for Britain.

Clegg has warned "there is a danger that the UK will be isolated and marginalised within the European Union", adding that Britain was "retreating further to the margins of Europe".

Merkel stressed that the new treaty was open to all members prepared to join – cautiously alluding to hopes that Britain might eventually follow suit – and should be merged with the EU's official treaties as early as possible.

There were no easy solutions and there would be setbacks in the long battle to tackle the crisis, Merkel said. "What is crucial is not the duration; what is crucial is whether we will allow setbacks to discourage us or not," she said.

Merkel vowed that Europe would not only overcome the debt crisis, but Europe would "emerge stronger from it than when it entered the crisis".