Reported by Bloomberg, Antony Broadbent, an analyst at Bernstein in London, said that write downs could total $3.3 billion at Barclays and $1 billion pounds at RBS.
A worst case scenario would see them announce $11.7 billion each, Mr Broadbent said.
Hikes in borrowing costs, after a collapse in the US sub-prime mortgage market, have led other banks to announce write downs and losses totalling almost $20 billion, as well as a run on Northern Rock.
Barclays and RBS have seen their combined value wiped by about $58 billion, exceeding the value reduction of $9.6 billion pounds for Barclays and $8 billion for RBS.
Mamoun Tazi, an analyst at MF Global Securities, said: "The market re-rated the shares on buybacks, but the recent ABN deal confirmed that the bank is likely to continue to do deals at the expense of shareholder value."