Greece warned it may opt out of a debt swap crucial to its second international bailout if too few investors rally behind it, raising the ante on the stricken country's 150 billion euro ($215 billion) lifeline.

The swap to shave 37bn euros off its debt is conditional on 90 per cent of private sector investors agreeing to the deal, the country said in its formal letter of inquiry to other governments.

Greece had previously set the threshold - which applies to the holders of its bonds maturing by 2014 and 2020 - as a target, not a condition.

'If these thresholds (or either) are not met, Greece shall not proceed with any portion of the transaction,' the letter said.

But Athens left itself some room for manoeuvre, saying that it would pull out of the deal if the take-up failed to satisfy its international partners, such as the European Union and the International Monetary Fund.

This would be the case 'if it determines, in consultation with the official sector, that the contribution of private sector creditors is insufficient to permit the sector to support the new multi-year adjustment programme.'

The letter follows signs that there had been a delay in making the formal offer to banks, sparking worries Athens was struggling to get enough investors to agree.

'It's a tactical move. They want to put pressure on private investors to take part,' said an official at a Greek bank. He added that reaching 90pc looked ambitious, though he expected the offer to go ahead even if the level wasn't reached.

The Institute of International Finance, a bank lobby group co-ordinating the talks, earlier said that between 60pc and 70pc of bondholders had signalled they would take part. More were likely to join, it said, once a concrete offer was made.

'By pinning it down so high, without any leeway in terms of a target range, it seems to me that they like to put some pressure on the system here,' said David Schnautz, strategist at Commerzbank, London.

The European Commission said that it believed enough private sector investors would take part to ensure the viability of the rescue package. A Paris-based banker said one drag on the deal was a row among euro zone governments about Greece's promise to provide collateral to Finland in return for its portion of the bailout.

Greece is aiming to get investors holding about 135 billion euros to take part, out of the roughly 150bn euros of bonds maturing by 2020.