UBS is poised to reveal further writedowns of up to $18bn and seek a capital increase of about SFr13bn ($13.1bn) only weeks after shareholders approved a similar-sized injection from outside investors.

Switzerland’s largest bank, which wrote off $18bn last year as it became the most serious European casualty of the US subprime mortgage crisis, has suffered from further falls in the value of mortgage and other credit securities during the first quarter, especially in March.