By Anirban Sen and Shankar Ramakrishnan

NEW YORK, Feb 28 (Reuters) - Family-owned candy giant Mars is preparing to sell bonds worth between $25 billion and $30 billion as soon as next week to help finance its takeover of Pringles maker Kellanova, according to people familiar with the matter, in a deal that would headline a $40 billion rush of acquisition financing bonds.

Banks led by Citigroup C.N and JPMorgan Chase JPM.N are preparing to market the bond sale to potential investors sometime next week, the sources said, cautioning that the timing of the bonds sale is subject to market conditions and could change.

Depending on its final size, the bonds offering could feature among the top 10 largest M&A financing deals in the investment-grade bond market since 2013, according to Informa Global Markets data.

Separately, design software maker Synopsys SNPS.O is preparing to sell between $10-15 billion of bonds to help finance its $34 billion takeover of Ansys, two of the sources added. Bloomberg earlier on Friday reported on Synopsys' bond sale talks.

The two deals combined could result in issuances worth about $40 billion next week, the sources said, boosting supply in an investment-grade bond market that is already seeing a flood of issuance driven by investor demand to lock in yields in a high-for-longer interest rate environment.

JPMorgan and Citigroup declined to comment. Mars and Synopsys did not immediately respond to requests for comment.

On Thursday, S&P Global Ratings downgraded its issuer credit rating on Mars to A from A+ noting the company expects to fund its Kellanova acquisition entirely with net debt.

"We do not forecast the company will restore leverage to below 3x or sustain discretionary cash flow $(DCF)$ to debt well above 10% until fiscal 2027. We expected the company to maintain those metrics for the 'A+' rating," S&P said.

(Reporting by Anirban Sen and Shankar Ramakrishnan in New York; editing by Chizu Nomiyama)

((Shankar.Ramakrishnan@thomsonreuters.com; +1 2017590156;))