Atlas Merchant Capital (AMC) is in talks to buy a £10m stake in the lender, according to a report by Sky News.
The move could allow app-based Tandem to tap into Diamond’s (pictured) heavyweight experience of UK retail banking.
Tandem claims to help people better manage their money – joining the likes of app-based offerings from Atom Bank and Monzo.
Tandem declined to comment on the reports.
Earlier this year the lender bought Harrods Bank to help it grow faster.
AMC has not responded to requests for comment.
Changes in the regulatory landscape lead to “unprecedented opportunities within the financial services industry,” according to AMC’s website.
Sky’s report claimed the talks were “very precarious” but a decision could be reached this week.