UPDATE 1-Ambac warns of default as bondholders organize

* Says could miss interest payment as soon as Q2 2010

* Says such an event could result in default

* Considers filing bankruptcy, raising capital

* Bondholders form ad hoc committee - sources

* Committee to push for prepack, debt-for-equity swap (Adds ad hoc bondholder committee, quotes, byline)

By Emily Chasan

NEW YORK, June 8 (Reuters) - Ambac Financial Group Inc ABK.N, the bond insurer whose toxic assets were seized by Wisconsin state regulators in March, said it could default on its loan obligations and was still considering filing a prepackaged bankruptcy.

The company, which has had trouble writing new business since losing its 'AAA' credit rating in 2008, said in a U.S. Securities and Exchange Commission filing on Tuesday that "as early as the second quarter of 2010" it may decide not to make interest payments on its debt, which could result in a default.

Bondholders who hold some of Ambac Financial Group's $1.24 billion senior debt, have formed an ad hoc committee and will try to push the company into a prepackaged bankruptcy, people familiar with the matter told Reuters. [ID:nWEN5654]

The bondholders' committee, which includes hedge funds Centerbridge Partners, Halcyon Capital Management, Mangrove Partners and Camden Asset Management, is looking to use a prepackaged bankruptcy to exchange their debt for equity in the company, the sources said. The sources declined to be named because the details are not public.

Such a swap could give the bondholders significant stock ownership of a reorganized Ambac and likely wipe out current equity, these people said.

Ambac said in the regulatory filing that it could consider raising additional capital, restructuring through a "prepackaged bankruptcy" or filing a traditional bankruptcy without agreements from creditors.

"While management believes that the Company will have sufficient liquidity to satisfy its needs through the second quarter of 2011, no guarantee can be given that it will be able to pay all of its operating expenses and debt service obligations thereafter, and its liquidity may run out prior to the second quarter of 2011," Ambac said in the filing.

The ad hoc bondholder committee has hired Morrison & Foerster as legal counsel and investment bank Lazard as financial adviser, the sources said.

Ambac said late on Monday that it has commuted its remaining $16.4 billion of exposure to collateralized debt obligations of asset-backed securities at its operating company Ambac Assurance Corp. The company will pay $2.6 billion in cash and issue $2 billion of surplus notes, as part of an agreement with counterparties, it said. [ID:nN07220391]

Ambac shares closed up 3.9 percent at $1.07 on the New York Stock Exchange on Tuesday.

Wisconsin state regulatorstook over roughly $64 billion of Ambac's worst assets in March. The insurer, along with rivals like MBIA Inc (MBI.N), has been battling crippling losses from risky mortgage securities amid the financial crisis. (Reporting by Emily Chasan; Editing by Richard Chang)