"I don't believe the agreement … has a chance" of passing the parliament before federal elections in Germany in the second half of 2013, Schröder said in an interview on the website of the 20 Minuten daily.
A tax deal between the two countries aimed at ending disputes over billions of euros hidden from German tax authorities in Swiss bank accounts is due to take effect on January 1st 2013, but still needs to be ratified by both parliaments.
Opposition lawmakers in the German upper house, including Schröder's Social Democrats, have threatened to block the tax deal.
Switzerland has meanwhile insisted the terms of the agreement, which have already been renegotiated once since it was signed last year, cannot be changed.
According to the agreement, Germans with non-declared funds in Switzerland will maintain their anonymity but their assets will be taxed by Bern, which in turn will transfer the revenues to Berlin.
Schröder said in Saturday's interview he could not understand why Germany should accept the continued secrecy, insisting Switzerland should at least pass on the names of Germans who move assets out of Swiss accounts to other countries considered tax havens before the deal takes effect.
"When wealthy people who benefit from Germany's excellent infrastructure, good schools and relatively good security think they do not need to contribute by paying taxes — or at least not all of their taxes — that is not OK," he said.
"And if other countries help German citizens to do so, that is not OK either," he stressed.
Swiss bank secrecy laws have reportedly allowed wealthy Germans to hide as much as €180 billion (219 billion francs, $236 billion) in Swiss bank accounts, beyond the gaze of German tax authorities.
Swiss President Eveline Widmer-Schlumpf,who is also Switzerland's finance minister, cautioned in July that if Bern and Berlin do not ratify the tax deal the existing procedure would continue, whereby Germany would have no option but to launch an independent inquiry for each case of suspected tax evasion.