The loss, which compares with a pre-tax profit of 7.4 million pounds the year before, is a further disappointment for the former world champion team, which is yet to score any points in Formula One this season.
However, Williams Grand Prix Holdings Plc said the loss was down to accounting technicalities which forced it to omit a chunk of Formula One income from its results. It said its business was on a sound footing.
The highlight on the track for Williams last season was a victory in the Spanish Grand Prix for Venezuelan driver Pastor Maldonado. The team finished 8th out of 12 in the constructors' championship.
"There is still a way to go for the team to get to where we should be, but improvements on previous seasons are evident," team principal and founder Frank Williams said in a statement.
Williams and other British-based Formula One teams such as McLaren are using the expertise from the sport to expand into other engineering and transport projects.
However, the core Formula One business still accounts for the majority of Williams's turnover, which rose 22 percent to 127 million pounds last year.
The team, which last won the Formula One constructors championship in 1997 when its drivers included Canadian Jacques Villeneuve - who won the drivers' title that year - received an additional 9.4 million pounds from Formula One's commercial rights holder last year after signing up to remain part of the world championship.
As that money would have to be repaid if the team pulled out of the sport before the end of 2015, Williams has had to defer including it in its accounts. It blamed the move for its slide into the red after a profit of 7.4 million pounds the previous year.
Williams Grand Prix Holdings was floated on the Frankfurt Stock Exchange two years ago. Former Executive Director Toto Wolff has said he plans to sell his 16 percent stake in the company after moving to rival Mercedes.