NCR has reopened the auction to sell itself — and the Blackstone Group is part of a group that plans to make a bid for the ATM maker, The Post has learned.

Having cancelled its investor day on Thursday, NCR sparked speculation it had reopened sales talks — and, in fact, it has, said to two sources close to the situation.

NCR shares spiked 7 percent on the news, to $26.46.

In fact, NCR has already signed a deal with a mystery buyer who has not yet lined up financing, a third source close to the situation said.

As reported exclusively by The Post on July 29, private equity firm Thoma Bravo dropped its bid to buy NCR — quickly ending what had been a belabored sales process.

Thoma Bravo had offered roughly $31 a share, and the seller was looking for $35.

Now, Thoma Bravo is deep in the action to buy Solera Holdings in a $3 billion auction and likely could not afford both, a source said.

The new auction process includes a lower asking price, sources said.

Blackstone, which is led by Chief Exec Stephen Schwarzman, declined comment. NCR did not return calls.