Leaders of the UK's flagship Reform congregation, the West London Synagogue, are investigating possible financial irregularities in excess of £100,000.
Losses of £144,000 have already been identified for last year, according to the newly available accounts. But the final total could be higher.
The accounts describe the amount involved as "significant" but the exact figure will not be known "until the investigation work is completed".
West London's trustees intend to "pursue vigorously for full recovery of any losses that have been incurred".
Members were recently notified by chairman Jonathan Lass that the possible irregularities uncovered in May and June this year "appear to go back several years".
Although the trustees wanted to advise members fully about the situation, "we are legally obliged not to comment or provide detailed information at this stage.
"We can say that the matter has been dealt with swiftly and all appropriate steps have been taken to safeguard and protect the synagogue's financial interests. We have advised the relevant authorities and our professional advisers." The possible irregularities contributed to the synagogue's deficit of £543,000 for 2009.
Simon Myers, executive director, said the shul had contacted the Charity Commission "and explained the background to the events we have uncovered and the steps we have subsequently taken. It is their view that we have done everything that they would have expected of us.
"Since June, we have been recruiting new members to the management team as well as adjusting procedures and practices, as necessary."
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