Switzerland, the United States and UBS agreed in August 2009 that the bank would identify 4,450 US clients suspected of tax evasion in an attempt to settle lawsuits by US authorities.

The deal needed parliamentary approval after the Swiss supreme court in January called its legal basis into question.

Although the two chambers, the senate and the National Council, have now both voted to endorse the agreement, they are at loggerheads over the need to allow it to be challenged by an optional referendum.

The senate maintains that the step is not necessary.

A spokesman for the parliament said Wednesday that the “conciliation commission” would meet early Thursday after fresh debates in each chamber failed to bridge the gap.

The commission, composed of 26 lawmakers from both houses of parliament, is designed to broker a compromise on such issues.

Under Switzerland’s complex legislative system, the “optional referendum” grants Swiss citizens 100 days to petition for a nationwide referendum to challenge some newly-passed legislation by collecting 50,000 signatures.

If approved, that would at least delay application of the UBS deal beyond a US deadline of August 19, the Swiss news agency ATS reported.

Parliament’s spokesman said that if the conciliation failed before the summer session closes at the end of this week, the agreement would effectively be buried.

The United States reiterated last week that it would revive the potentially damaging litigation against UBS if Switzerland reneged on the agreement, which opened a corner of the country’s coveted banking secrecy.

The National Council joined the senate in voting in favour of the US-Swiss deal itself on Wednesday, after initially opposing it.