ZURICH (Reuters) - UBS might need to retain $10 billion to $15 billion in excess capital after Switzerland's government this week laid out plans for tougher capital requirements for the enlarged lender, Autonomous Research estimated on Thursday.

In its base case, UBS would need 200 to 300 basis points more in common equity tier 1 ratio (CET1 ratio), a measure of a bank's resilience, which "would require the retention of around $10-15 billion incremental CET1 capital in coming years," banks analyst Stefan Stalmann wrote in a note to clients.

The need to retain such an amount could "seriously dent" current expectations for UBS buybacks of its shares, he said.

In a worse case "headwind", UBS would face up to 700 basis points more for its CET1 ratio, Stalmann said, although he cautioned that his estimates were based in part on guesswork because the Swiss government had not quantified the extra capital it believed the enlarged UBS would need to hold.

UBS declined to comment.

Stalmann, who said he maintained a neutral rating on UBS stock, said the government's recommendations, which form part of Switzerland's efforts to shield the country from a repeat of the collapse of Credit Suisse, would lead to uncertainty.

That is because the government only plans to finalise its regulatory plans in the first half of 2025 which, he said, "is bound to create a lengthy period during which investors, but also UBS itself, will face elevated uncertainty about medium-term capital planning and payout prospects."

UBS shares, which have soared since the bank agreed a rescue takeover of Credit Suisse a year ago, fell sharply on Wednesday and were down another 2.5% on Thursday, in line with declines across European banking stocks.

Switzerland's finance ministry said on Wednesday its "too big to fail" recommendations envisaged tougher capital requirements for UBS and other systemically important banks following the rescue of Credit Suisse.

But UBS could take years to feel the bite as the plans laid out by the government were light on detail and heralded a tortuous political process to enshrine them in law.