Dutch secretary of state Tineke Huizinga urged Bart Jacobs and Wouter Teepe not to publish any secrets of their hack that may lead to smartcard systems being abused.
The duo, from Radboud University in Nijmegen, had planned to publish a paper on the subject at the Esorics security conference in October.
The Dutch Government was going to introduce a smartcard payments system based on the same chip as Oyster, called Mifare. But it postponed the 1bn euro project earlier this year after Jacobs and Teepe revealed they could carry out the hack.
Jacobs defended his research. Quoted in the Dutch ICT publication WebWereld, he said he was providing mathematical analysis and not attack code. "It requires a lot of expert work to transform the analysis from the Esorics paper into a working device for performing attacks on card installations," he said.
He added that other groups may have written tools to carry out such an attack and distributed them on the internet. He could offer no additional comment at the time of writing.
Academics from University College London have also broken the Mifare cipher. UCL's Nicolas Courtois said he could carry out his attack in just 12 seconds.
Transport for London maintains that making a clone of an Oyster card is illegal.