Boro's results for the year up to June 3, 2020 also showed a massive drop in income from £55m to just £19m, with the double impact of the ending of the club's parachute payments following relegation from the Premier League in 2016-17 and the financial ramifications of Covid-19 hitting the club hard.
Boro posted a pre-tax loss of £35.5m, a big drop from a profit of £2m in their previous set of accounts.
The club's highest-ever loss prior to the latest figures was a record deficit of £31.9m in 2015-16.
The accounts pointed to the continued financial support of Gibson, who said that the club’s biggest financial risk is “failing to perform competitively in the league in which it competes”.
The accounts stated: “The going concern basis of the company depends on the support from the Gibson O’Neill Company Limited, the ultimate parent undertaking, who will continue to provide that support, if required, to the company for the foreseeable future.
“The club will continue to keep control of operating costs and make as much income as possible available to invest in the football team.
“The club will continue to strive for progression and promotion to the Premier League.”
The figures showed that the club's wage bill fell from £40m to £31m, while revealing that the club received £581,000 from the Government's furlough scheme during the period.
Profit in players sales fell to £3.5m from £33m and there was a huge fall in broadcasting income from £40.6m to £8.3m following the ending of parachute payments from the Premier League.
Sponsorship income, gate receipts - with seven games played without fans in the second half of 2019-20 - and merchandising sales also took a hit.
Meanwhile, Middlesbrough boss Neil Warnock has been charged with improper conduct by the Football Association over his post-match comments following his side’s controversial Championship defeat at Swansea.
An FA statement read: "Neil Warnock has been charged with a breach of FA Rule E3 in relation to Middlesbrough FC’s match against Swansea City FC in the EFL Championship on Saturday March, 6 2021.
"It is alleged that comments made by the manager during a post-match interview constitute improper conduct as they questioned the integrity of the match official and/or implied bias and/or were personally offensive and/or bring the game into disrepute contrary to FA Rule E3.1.
"Neil Warnock has until March 22, 2021 to provide a response to this charge."