Celtic made a post-tax loss of £12.6m last season as the coronavirus pandemic and reduced transfer income hit home.
The club had made a small pre-tax profit the previous year which resulted in a total loss of £368,000.
Revenue was down by 13 per cent to £60.8m for the year ended June 30, 2021.
Operating expenses also fell, by eight per cent, but were still significantly higher than revenue at £74.4m.
The club made a loss on their transfer activity, bringing in £9.4m and spending £13.5m. The previous season's activity had been boosted by the sale of Kieran Tierney. The balance sheet showed "other income" of £5m.
The balance sheet showed a £630,000 spend on "settlement agreements on contract termination" for Celtic, who saw manager Neil Lennon step down in February.
They still had £16.6m cash, net of bank borrowings, at the end of their financial year, which allowed them to spend money in the transfer market this summer, although they subsequently sold Odsonne Edouard, Kristoffer Ajer and Ryan Christie for a combined total of around £30m.
Chairman Ian Bankier said the losses were "driven by revenue attrition and significantly lower gains on player trading, compared to the prior financial year".
"In the face of this adverse swing in financial performance, we are satisfied that we took sufficient and appropriate steps to mitigate the losses and control costs in the business," he added.
"The persisting trading restrictions from Covid-19 translated into lost earnings and, taking account of the seasonality in our trading, this was the key factor in the widening of our losses in the second half of the financial year.
"Conditions have improved markedly since the year end and we were delighted to welcome our fans back in July 2021.
"Although our stadium has been operating at near full capacity, recently announced Scottish Government restrictions on large venues will be a further challenge.
"Whilst we look forward with optimism to a more normal operating environment, we are mindful of the inherent risk of the pandemic continuing to affect public health."
Celtic bought Albian Ajeti, Vasilis Barkas and David Turnbull during the 12-month period while there were further expenses for signing Mohamed Elyounoussi, Shane Duffy and Diego Laxalt on loan.
Their outgoing players included Jack Hendry, Jeremie Frimpong and Patryk Klimala with further payments for the latter two likely to come.
Bankier said: "Our strategy of balancing player development and player trading is fundamental to our self-sustaining business model.
"In particular, the disposal of the registrations of Odsonne Edouard and Kristopher Ajer demonstrates our core strength of being able to transform young talented players into seasoned professional footballers.
"In turn, we invest the proceeds of these transactions back into the first-team squad to enable us to continue to develop our squad and to challenge for future honours."
There were no further details on the recent departure of Dominic McKay as chief executive, around 10 weeks after he officially succeeded Peter Lawwell.
Acting chief executive Michael Nicholson wrote in his review of the year: "Over the recent years we have invested heavily in our football department so that it can perform at the highest levels in supporting our playing squads and our player identification and development strategy.
"That investment fed in to one of the most successful periods on the pitch in the club's history, and we will continue to devote substantial investment to these key areas, our objective being to remain at the forefront of the modern game.
"A number of evolutionary initiatives are currently under way, to build on that success and to continue to take the club forward."