CHICAGO, April 29 (Reuters) - The union representing flight attendants at bankrupt United Airlines on Friday reiterated a threat to strike if the No. 2 carrier ends their pension plans.

United, a unit of UAL Corp., and federal pension insurers last week reached a settlement, clearing the way for the carrier to shed its four employee retirements plans. A bankruptcy judge was expected to rule on the settlement on May 10.

"If they proceed with termination and destroy our retirement security, they leave us no alternative but to strike," said Greg Davidowitch, president of AFA's United master executive council.

The Association of Flight Attendants expressed outrage at the settlement, which, if approved, would turn the pensions over to the Pension Benefit Guarantee Corporation.

The pension agency said the difference between promised benefits and assets in the four United plans is $9.8 billion. The government will guarantee retirement benefits totaling $6.6 billion.

AFA members in December authorized the union to call for strike activities if the carrier terminates their collective bargaining agreement.

United Spokeswoman Jean Medina said the carrier believes a labor strike under the circumstances would be illegal and that the union should spend its time pursuing consensual agreements with the airline.

United, battered along with the rest of the industry by soaring fuel costs and weak revenues, has said it needs to achieve additional annual labor savings in order to emerge from Chapter 11 protection.

The carrier also has said that voiding and replacing its pension plans would give United an average savings of $645 million a year over five years.

Friday's renewed threat by the AFA is the latest assault by the union, which has been especially vocal in its opposition to pension termination. The AFA said its strikes would be intermittent and designed to surprise.