The pension scheme for university academics would face a deficit of £23bn if it had to be rescued by a government-backed pension lifeboat, the scheme's annual accounts show.
The figures will stoke a fierce debate about the Universities Superannuation Scheme's (USS) financial position.
It has assets of more than £60bn and provides retirement incomes and savings for about 200,000 UK university staff.
The USS said it funds the scheme in a "fair and balanced way".
In February and March 2018, academics at 64 universities took industrial action over plans by their employer Universities UK to reduce future benefits and introduce a less-generous pension arrangement.
The University and College Union said the new plans would leave them up to £10,000 a year worse off in retirement, and younger staff would be worst affected, with some losing up to half their pensions.
USS's accounts for the year to the end of March 2019, which were published last week, give several different figures for the size of the scheme deficit.