The investigation is expected to finish at the end of this month

Mr Rove's lawyer says his client is happy to co-operate and he says he has received no information to suggest that he is facing prosecution.

The name of CIA agent Valerie Plame was revealed by a journalist in 2003.

Ms Plame's husband is a former diplomat who criticised President George W Bush over Iraq.

It was alleged a White House source leaked her identity.

Indictment

The special prosecutor investigating the case called on Karl Rove to give more evidence and informed him that he was not necessarily going to escape prosecution.

However, his lawyer said his client had not been told he was likely to face indictment.

Mr Rove has already appeared in front of the grand jury, and his return at this late stage in the investigation is unusual, correspondents say.

As such, the mood surrounding this inquiry has suddenly changed, in a manner deeply worrying for the Bush team, says the BBC's Justin Webb in Washington.

It is not known when Mr Rove will appear before the grand jury.

The White House had long maintained Mr Rove had nothing to do with the leak, but reporters have since said he talked about the agent although not by name.