Advisers to General Motors Corp bondholders representing $27 billion in the automaker's debt urged investors on Friday to support a debt swap negotiated over the past week with the Obama administration.

Bondholders have until Saturday to register their support for the terms of a deal that would give them up to 25 percent of a reorganized GM. That offer is contingent on the U.S. Treasury determining that enough investors have signed on in support.

Investors representing at least 35 percent of GM's bonds are expected to support the sweetened offer from the U.S. Treasury, which will be the automaker's largest shareholder and creditor.

In a conference call open to GM bondholders, advisers to an ad hoc committee representing institutional investors urged other bondholders to offer their support for the deal.

GM's bondholders have been advised by law firm Paul, Weiss, Rifkind, Wharton & Garrison LLP and financial adviser Houlihan Lokey Howard & Zukin Capital Inc.

Andrew Rosenberg, the legal adviser to the GM bondholder committee, said investors should understand the automaker's intent to file for bankruptcy on Sunday or Monday.

GM, assuming its board approves, will file for bankruptcy Sunday, Monday of next week, he said.

GM bondholders had been in intermittent talks with members of the U.S. Treasury's autos task force overseeing GM's restructuring since early in the year.

Earlier this week, bondholders rejected a proposal that would have given them a 10 percent stake in the reorganized GM,

and a tender offer for new stock in the company was withdrawn.

In a sweetened deal, the U.S. Treasury and GM offered bondholders warrants for another 15 percent stake under a proposed deal that could be rescinded if too few bondholders sign on to support it.

Rosenberg said the new deal terms had been negotiated in the last week.

Eric Siegert, who has been the financial adviser to the bondholder committee, said the new deal represented a total government investment in GM of nearly $60 billion.

For the government to be repaid in full, GM would have to have an enterprise value of $69 billion based on its expected 72.5 percent stake in the company, Siegert said.

By the time GM reached that market value, bondholders would have recovered $14 billion from their new equity, he said.

That's a very, very unusual result that you have a junior creditor ... where a company is in bankruptcy (and) where they receive that much value before a senior creditor is paid in full, Siegert said.

He added: The federal government will not be paid in full on its equity investment until the value of General Motors reaches $69 billion.

Bondholder advisers said they would not disclose the names of investors who supported the U.S. government's plan for GM, which would include a quick sale of its best assets to a new company funded by the U.S. Treasury.

Later on Friday, a group of individual GM bondholders called GM Bondholders Unite said White & Case bankruptcy partner Thomas Lauria will represent their group. Lauria was also representing Chrysler bondholders, the group said.

(Reporting by Kevin Krolicki, David Bailey, Soyoung Kim, Poornima Gupta, Walden Siew and Elinor Comlay, editing by Matthew Lewis)