The Securities and Exchange Commission filed an emergency request Tuesday to put its securities fraud lawsuit against Citigroup Inc on hold, saying a failure to delay the case would derail its proposed settlement with the bank and threaten "irreparable" harm.

The filing with the 2nd Circuit Court of Appeals in New York seeks an expedited appeal of a Nov. 28 decision by District Judge Jed Rakoff that harshly rejected the SEC's $285 million accord with Citigroup.

Rakoff said that because the SEC did not require the New York-based bank to admit or deny its charges, he had no way to know whether the settlement was adequate.

But the SEC said the ruling was "legal error," at odds with decades of court decisions allowing such settlements and letting investors get faster recoveries, and could affect its ability to reach similar accords with other companies.

According to Tuesday's filing, Rakoff in a teleconference set a January 3, 2012 deadline for Citigroup to "answer" the SEC complaint, 27 days sooner than federal rules require.

An answer can force Citigroup to deny some or all of the SEC allegations, or seek to dismiss the case entirely.

But this "will disrupt a central negotiated provision of the consent judgment pursuant to which Citigroup agreed not to deny the allegations," the SEC contended. "The parties will not be able to return to their initial bargaining positions should this court ultimately reverse the district court."

The SEC said Citigroup has agreed to its request to put the case on hold and allow an expedited appeal.

Announced on October 19, the settlement was intended to resolve charges that Citigroup sold $1 billion of risky mortgage-linked securities in 2007, without telling investors that it was betting against the debt. Investors lost more than $700 million, the SEC has estimated.

The $285 million payment was to include $160 million of disgorged profit and fees, $30 million of interest and a $95 million civil fine. Rakoff called the penalty "pocket change" for Citigroup, the third-largest U.S. bank.

But the SEC has said the law limits the sums it can recover. It has asked Congress for authority to seek larger penalties in corporate cases.

Rakoff has set a July 16, 2012 trial date. One Citigroup employee, director Brian Stoker, was also charged by the SEC, and has been contesting those charges.

The case is SEC v Citigroup Global Markets Inc, 2nd U.S. Circuit Court of Appeals, No. 11-05227.

(Reporting by Jonathan Stempel in New York)