The latest accounts show it had valued its stake in Thames’ parent Kemble at around £260m in December, which represented a 62.8pc fall over the last financial year.
However, this has since fallen to zero after Omers joined other shareholders in cutting off funding from Thames, claiming it had been rendered “uninvestable”.
This shortfall led to Kemble, Thames’ parent, defaulting on its debts earlier this year.
Omers’ bosses said these events, which took place after its year-end in December, have led to “a full write-off of the investment”.
This marks the latest blow for Thames Water, which is at risk of collapsing under the weight of its £18bn debt pile.
Government-backed contingency plans are being drawn up should this happen, which could cost the taxpayer billions of pounds.