The banks leading HBOS’s £4bn (€5bn) rights issue, which look like they will be saddled with unwanted shares after the UK lender's stock price slumped below their rights price, are facing the additional threat of sub-underwriters exercising an option to offset their exposure, according to bankers.

Dresdner Kleinwort and Morgan Stanley face the prospect of having to hold most of the unsold shares in the rights issue depending on level of flexibility contained in their agreements with sub-underwriters, who are understood to be HBOS’s large institutional shareholders.