* ACI cash-and-stock offer valued at $540 mln
* Says offer gives more value than S1's Fundtech merger plan
* S1 shares up 32 percent, Fundtech shares down 9 percent (Rewrites throughout, adds details, updates share movement)
By Jochelle Mendonca
BANGALORE, July 26 (Reuters) - ACI Worldwide Inc , which makes e-payment processing software, offered to buy peer S1 Corp for $540 million, potentially derailing S1's planned buy of Fundtech .
Last month, S1 Corp agreed to acquire Israel's Fundtech for about $700 million in an all-stock deal.
"We are confident S1 shareholders will share our strong belief that ACI's premium proposal is superior to and provides S1 shareholders with significantly greater value than the pending transaction with Fundtech," ACI Chief Executive Philip Heasley said in a statement.
In a letter to S1's board, Heasley said S1 must open its books and negotiate with ACI, in accordance with the 'superior offer' provisions in its merger agreement with Fundtech.
ACI added that it was "prepared to do what is necessary" to make the deal happen.
The company has offered $9.50 per S1 share, a premium of 33 percent to S1's stock close on Monday, and said the consideration in the proposal consists of 40 percent in ACI stock and 60 percent in cash.
S1's shareholders cheered the bid, sending its share to $9.45 in pre-market trade. Fundtech's shares, however, were down nearly 9 percent.
ACI expects the proposed transaction to add to its full-year earnings in 2012.
It said the offer is not subject to any financing condition, and it has secured committed financing from Wells Fargo Bank for the cash portion of the transaction.
Separately, ACI Worldwide reported a quarterly profit, and raised its full-year outlook.
Shares of ACI had closed at $35.70 on Nasdaq. (Reporting by Brenton Cordeiro and Jochelle Mendonca; Editing by Joyjeet Das)