Wulff, once a close ally of Chancellor Angela Merkel, claimed he knew nothing about Olaf Glaeseker going on free holidays, but this may have been a lie.
Wulff resigned in February after failing to convincingly brush off a string of unsavoury, money-related allegations – including that in 2008 while he was Lower Saxony state premier, he failed to declare a soft loan of €500,000 from a prominent regional businessmen.
Glaeseker was his spokesman during that time, and is tied up in an investigation about his allegedly corrupt relationship with event organiser and friend Manfred Schmidt.
Glaeseker organised large donations for Schmidt's business – and is alleged to have accepted 17 holidays and 18 flights in return.
Wulff said he had little or no idea about his spokesman's holidays and could not reach him while he was away. Yet a report in Sunday's Bild newspaper said Wulff's main assistant at the Lower Saxony chancellery was adamant that Wulff often spoke with Glaeseker while the latter was on holiday.
And Glaeseker has, according to Der Spiegel, handed prosecutors his wife Vera's diary in which she wrote who her husband spoke with while they were away together.
The nickname he had given Wulff, Knödel meaning dumpling, featured in the diary.
Wulff and Glaeseker are being investigated in separate cases, both on suspicion of corruption.