As part of its half-year results on Friday, the group is poised to put a £145m value on its 20 per cent stake in the lender, down from £185m. The writedown implies a total value of £725m.
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The expected announcement, first reported by Sky News, will mark the second time this year that Co-op Group has slashed the value of its shareholding.
In April, the group cut the value of its stake by £39m, from £224m to £185m.
The decision by the Co-op board is set in the context of banking industry uncertainty, with interest rates at record lows.
In 2013, the Co-op Bank nearly collapsed when a £1.5bn hole in its capital was discovered following losses from problem real estate loans.
Co-op Group declined to comment.