West Ham lost more than £540,000 per week last season before tax following significant investment on transfers and wages, coupled with a decline in profits from player sales.
According to the club's financial accounts for the 2018/19 season, West Ham swung from a pre-tax profit of £18.3million to a pre-tax loss of £28.2million.
Revenues were up more than £15million year-on-year to £190.7million, driven by an increase in broadcasting payments after a 10th place finish last season.
But a litany of first-team signings, including Felipe Anderson, Issa Diop, Andrey Yarmolenko, Jack Wilshere, Lukasz Fabianski and Ryan Fredericks, had a significant bearing on costs.
Lucas Perez, Fabian Balbuena, Xande Silva and Carlos Sanchez also joined the Hammers, as the club revealed an investment of £107.9million on transfers under former manager Manuel Pellegrini.
As a result, wages leapt from £106.6million to £135.8million, pushing the club's revenue/wage ratio up from 60.8% to 71.2%.
UEFA recommends clubs to keep this ratio below 70% for long-term sustainability.
Meanwhile, the annual amortisation charge - the non-cash accounting cost of signing players, where the cost is written off over the length of the contract - jumped considerably, from £41.0million to £57.4million.
In cash terms, West Ham's transfer activities saw future payments due on player signings also jump.
At the end of 2018/19, the club owed a little under £87million to other clubs.
In return, West Ham were owed only £16.7million.
West Ham's bottom line was also impacted by a material decline on profits on player sales.
The club offset its operating losses by recording a £10.5million profit on disposals, mainly Cheikhou Kouyate's move to Crystal Palace.
Meanwhile, a further £2.2million was recognised in loan fees, including Edimilson Fernades to Fiorentina, Reece Oxford to Augsburg and Sam Byram to Nottingham Forest.
In 2017/18, the club declared a much higher profit of £29.9million on player sales, including Andre Ayew's big money move to Swansea.
