Twenty-five of the EU's 27 member states have agreed to join a fiscal treaty to enforce budget discipline.
The Czech Republic and the UK refused to sign up. UK Prime Minister David Cameron said his government would act if the treaty threatened UK interests.
He still has "legal concerns" about the use of EU institutions in enforcing the fiscal treaty, he said.
The Czechs cited "constitutional reasons" for their refusal, France's President Nicolas Sarkozy said.
Czech President Vaclav Klaus, a Eurosceptic, may be reluctant to sign the treaty, analysts say.
The goal is much closer co-ordination of budget policy across the EU to prevent excessive debts accumulating.
Germany - the eurozone's biggest lender and most powerful economy - was particularly keen to get a binding treaty adopted to enforce budget rules.
The treaty will empower the European Court of Justice to monitor compliance and impose fines on rule-breakers.
The treaty also spells out the enhanced role of the European Commission in scrutinising national budgets.
The Czech Republic is not yet in the euro, but like the other new EU member states it is committed to joining.
European Union leaders also discussed ways to stimulate economic growth despite the stringent austerity budgets in many countries - and focused on how to reduce unemployment across the eurozone.
The UK and Denmark are the only states with explicit opt-outs from the euro.
Mr Cameron said "it's good that the new treaty is absolutely explicit and clear that it cannot encroach on the competences of the EU".
"They must not take measures that in any way undermine the EU single market," he said, adding: "we'll be watching like a hawk".
He insisted that the treaty would impose "no obligations on the UK".
Mr Cameron used his veto last month to opt out of the treaty, arguing that the UK needed to keep its authority over financial services in the City of London.
The eurozone crisis dominated Monday's summit, with debt-laden Greece still at risk of defaulting.
A general strike in Belgium, paralysing transport, reminded EU leaders of public discontent with austerity as they arrived for the summit.
The talks also concentrated on reducing unemployment, which is averaging 10% across the eurozone, though youth unemployment is often much higher.
There are fears that wide-ranging budget cuts will harm enterprise and training.
The leaders discussed measures to support small and medium-sized enterprises (SMEs), many of which complain of excessive administrative costs imposed by Brussels.
In a joint statement on economic growth they noted that cutting budget deficits was "not in itself sufficient".
"We have to modernise our economies and strengthen our competitiveness to secure sustainable growth," the statement said.
The EU will help to fund schemes to get young people into work or training in member states with the highest youth unemployment levels.
They pledged to speed up measures to develop the EU single market, including:
- agreement on a common EU patent system by July;
- better targeting of EU funds towards SMEs;
- national legislation to create a functioning single market in services and energy.
The European Commission says 82bn euros (£69bn; $107bn) of EU money is available for countries to spend on projects to boost jobs and growth.