ZURICH, Nov 28 (Reuters) - Wealth manager Julius Baer's shares slumped another 5% on Tuesday, bringing its total decline since the start of last week to more than 20%, on fears over its exposure to struggling real estate group Signa.

Signa, controlled by an Austrian magnate, has borrowed heavily from banks including Julius Baer, which on Monday disclosed it had an exposure of more than 600 million Swiss francs ($682.6 million) to the conglomerate and its entities.

The exposure is the largest in the bank's private debt loan book, which amounted to 1.5 billion francs at the end of October, it said. Julius Baer declined to comment further on Tuesday.

Shares were down 5.6% by 1505 GMT to trade at their lowest this year, after Morgan Stanley analysts downgraded the stock to underweight.

"Given revenue headwinds, credit quality issues and low capital returns, we see better risk-reward elsewhere in the sector," the analysts said.

Baer has in November already booked 70 million francs in provisions against its credit portfolio.

Morgan Stanley analysts said they are expecting Julius Baer to take an additional 50 million francs of provisions in 2024. Vontobel analyst Andreas Venditti puts the number far higher and thinks the Swiss bank could write down half of its 606 million Swiss franc exposure to Signa.

"In our understanding, 'private debt' does not relate to (lower-risk) mortgage exposure," Venditti wrote in a note to clients.

"Julius Baer's exposure is therefore high-risk, given the very difficult financial situation of the Signa Group."

($1 = 0.8790 Swiss francs) (Reporting by Noele Illien; Editing by Jan Harvey)