Royal Bank of Scotland plc (NYSE: RBS), is the "most vulnerable" bank in Europe and may have to raise billions of dollars to strengthen its capital ratios, warned analysts at Credit Suisse.

The 83-percent state-owned bank, whose long-term credit default rating has already been cut to A from AA- by Fitch Ratings, might have to come up with £16.9 billion ($26.7 billion), Credit Suisse said. That figure exceeds that of any other major British or European lender.

Dozens of British banks, including RBS, released results of new stress tests form to the European Banking Authority on Thursday.

Credit Suisse has estimated that about two-thirds of these banks might fail the latest stress tests, adding that the combined capital shortfall could total £193 billion ($305 billion).

"RBS appears to be the most vulnerable although the company has said that the methodology, especially the calculation of trading income, is especially harsh for them," said Credit Suisse.

According to a report in Britain’s Daily Telegraph newspaper, such a capital shortfall might lead RBS to become fully-nationalized.

However, according to InvesmentWeek, RBS is not in such great danger.

HSBC said in a research note that RBS is unlikely to face capital shortfall.

An analyst at Evolution Securities, Ian Gordon, said last week in a note that the idea of RBS needing to raise more capital was "comical".

"Many European banks do need material recapitalization. RBS, for all its many faults, does not," she said.

RBS shares are trading 1.3 percent higher in mid-morning trading in New York.