The Olympic Park Orbit tower lost £520,000 in 2014-15, according to the annual report of the local development agency which oversees the attraction.
The London Legacy Development Corporation (LLDC) previously forecast a profit of £1.2m.
Labour leader at the London Assembly, Len Duvall, called it a "pointless monument at vast taxpayer expense".
The LLDC defended the project, saying it was "one of the standout successes of the 2012 Games".
The tower, which was mostly funded by the ArcelorMittal steel company and received £3m in public money, opened in 2012 as an observation tower.
It closed after the Olympics while it was adapted for long-term use, and reopened in April last year.
Mr Duvall associated the attraction's fortunes with the mayor: "It's clear that this is yet another Boris Johnson vanity project of towering proportions.
"Instead of paying back some of the £3m of taxpayer investment the Orbit is actually losing £10,000 a week, an awful record even by Boris' standards."
The BBC has contacted the mayor's office for comment.
A spokeswoman for LLDC said: "The ArcelorMittal Orbit was one of the standout successes of the 2012 Games and has seen almost 200,000 visitors since reopening in 2014, which is a tremendous achievement."
The LLDC has revised its estimate of visitor numbers down from its original forecast of 350,000 a year to 150,000, but said it was "constantly looking at ways to enhance the experience to attract more people".
Plans to add a giant slide down the length of the tower were approved this summer and its construction is expected to finish in 2016.