Blau allegedly used classified documents stolen by journalist Anat Kam during her IDF service to write several news items. A gag order on the affair was lifted earlier Thursday.
The Haaretz reporter has been abroad for the past several months, as he engaged in negotiations with the Shin Bet and State Prosecutor's Office aimed at securing classified documents he still possesses.
Shin Bet officials charged earlier that Blau reached an agreement with them and even handed over his personal computer, but failed to hand over all the documents he received from Kam.
Officials have refrained from revealing the exact nature of legal measure they intend to take in order to ensure Blau returns to Israel for questioning, saying an extradition motion is a concrete possibility.
Nevertheless, Attorney Devora Chen, former deputy attorney general for security related affairs, said the matter was very complex, explaining that British judiciary tends to look at political aspects of offences. In the case in question, where the person of interest is a journalist who has criticized the defense establishment, an extradition might prove doubly difficult, she said.
Both the State Prosecutor's Office and the Shin Bet claim that the documents in Blau's possession include highly sensitive information. Shin Bet Chief Yuval Diskin, who held a special media briefing earlier Thursday, characterized the material in question as having the potential to "gravely damage state security and endanger the lives of both soldiers and Israeli civilians."
However, Blau's attorney claimed that the Shin Bet reneged on the agreement with his client.
Meanwhile, Ynet discovered that this is not the first such entanglement for Blau. Almost eight years ago, a female soldier serving in the Gaza Strip was caught and punished after leaking documents to the journalist.