The threat of a Greek debt default undermining the euro overshadows an EU summit getting under way in Brussels. The 27 EU leaders are expected to reaffirm their determination to defend the single currency. But the BBC\'s Europe editor says there is little they can do until the Greek parliament votes on a new package of tough austerity measures next week. Many economists think a Greek default is inevitable sooner or later - and that would hit confidence in the euro. Prime Minister David Cameron is likely to insist again that the UK - which did not contribute to the first Greek bail-out last year - should not be involved in any second Greek rescue, our Europe editor Gavin Hewitt says. Migration pressures are also on the agenda at the two-day summit. The key question is whether, following an influx of migrants from North Africa this year, the EU\'s open borders policy should in exceptional circumstances be suspended. Leaders are also expected to endorse and set a date for Croatia\'s accession to the EU - likely to be July 2013. Greece\'s EU partners are anxious to see the Greek parliament approve a five-year austerity package of tax increases and spending cuts worth 28bn euros (£25bn; $40bn). Greece will not get the next instalment of its 110bn-euro bail-out unless it adopts the measures. Yet many Greeks are angry, believing that their country is being sacrificed for the sake of the euro, and they have held mass protests and strikes against the EU\'s stringent bail-out conditions. Eurozone ministers recognise that Greece will need another big EU lifeline. On Sunday they agreed to put together a second bail-out package worth 120bn euros to fund Greece into late 2014. In an interview on the BBC\'s Hardtalk programme French Foreign Minister Alain Juppe spoke of \"a very strong determination in the [EU] member states to save what has been done in the past 50 years\". \"We\'ve no choice. If the eurozone disappeared the EU itself would be in danger - we can\'t afford such a situation,\" he said, insisting that \"there will not be any default of Greece\". From BBC News