* Prosecutors search two German Opel sites
* Transport Ministry says defeat device found in April
* Ministry says recall of around 100,000 Opel cars imminent
* Opel says will legally challenge a mandatory recall
* Opel denies using defeat device (Adds Opel statement)
FRANKFURT, Oct 15 (Reuters) - Germany's Transport Ministry said on Monday it would order roughly 100,000 Opel vehicles to be recalled as part of an emissions probe, after prosecutors searched the carmaker's offices earlier in the day.
German motor vehicle authority KBA found four software programmes capable of altering vehicle emissions in 2015, and ordered Opel to implement a software update in cars to remove them, Germany's Transport Ministry said in a statement.
"After a fifth software device was discovered in early 2018, which KBA found to be illegal, there is currently an official hearing going on with the goal of imposing a mandatory recall for the models Cascada, Insignia and Zafira," the ministry said in a statement.
The transport ministry said Opel had dragged its feet on the hearing. The KBA had told Frankfurt prosecutors about the software device in April, it said.
"The official recall of the affected roughly 100,000 vehicles will take place shortly," it added.
Opel said in a statement it rejected any accusation of using illegal defeat devices that can manipulate exhaust emission tests, also denying it was procrastinating.
"Should (a mandatory recall) be ordered, Opel will challenge it legally," said the carmaker .
German prosecutors searched offices at Opel's sites in Ruesselsheim and Kaiserslautern earlier on Monday.
The Frankfurt prosecutor's office said it was probing 95,000 vehicles equipped with Euro 6d engines.
PSA Group, which owns the Opel and Vauxhall brands, declined to comment.
The Opel Insignia, Zafira and Cascada were developed when Opel and its sister brand Vauxhall were still owned by General Motors. General Motors sold Opel to PSA Group in 2017.
Opel admitted in 2016 that its Zafira model contained engine software which switched off exhaust emissions treatment systems under certain circumstances. Opel insisted at the time that it was making use of a legal loophole. (Reporting by Hans Seidenstuecker and Irene Preisinger, additional reporting by Laurence Frost in Paris; writing by Edward Taylor; Editing by Maria Sheahan, Jan Harvey and Mark Potter)