(Adds context, company responses)
SYDNEY, April 19 (Reuters) - A consortium backed by U.S. private equity firm KKR & Co offered A$6.15 billion ($4.65 billion) cash for Australian lottery operator Tatts Group Ltd, threatening an agreed merger with Tabcorp Holdings Ltd, two sources familiar with the situation said on Wednesday.
The sources, who were not authorised to speak publicly, told Reuters a letter offering A$4.21 a share had been sent to the Tatts board on Tuesday evening.
Representatives of Tatts, Tabcorp and the KKR-backed Pacific Consortium declined immediate comment.
Tatts in October agreed to accept a cash-and-scrip proposal from betting group Tabcorp to form an Australian gambling powerhouse.
Tatts shares closed at A$4.35 a share on Tuesday. The Tabcorp offer, which has yet to receive competition approvals, values Tatts shares at A$4.21 based on Tabcorp's closing price on Tuesday.
The Pacific Consortium, which in December made a takeover offer rejected by the Tatts board, also includes Macquarie Group, Morgan Stanley Infrastructure and First State Superannuation Scheme.
Tabcorp in November purchased a 9.9 percent stake in Tatts to make it harder for an interloper to block their agreed merger.
($1 = 1.3226 Australian dollars) (Reporting by Jamie Freed; Editing by Andrew Hay and Grant McCool)