The Pension Benefit Guaranty Corp. has pledged to oppose the reorganization of UAL Corp., calling the plan “unconfirmable” because it violates an agreement the parent of United Airlines Inc. struck when it terminated its pension plans last spring.

In a filing late Thursday with the U.S. Bankruptcy Court for the Northern District of Illinois, the PBGC said UAL’s plan would place “onerous restrictions” on the pension program’s ability to sell shares it is to receive in the reorganized company. Elk Grove Township, Ill.-based UAL and the PBGC reached a settlement earlier this year that called for the government entity to assume more than $6 billion in United obligations in exchange for common and preferred stock in the company.