TOKYO (Reuters) - Four of Japan's top property and casualty insurers and other financial firms plan to sell around 500 billion yen ($3.1 billion) of shares in Honda Motor , three people said, as the unwinding of cross-shareholding practices accelerates.

Tokio Marine Holdings Sompo Holdings and two units of MS&AD Insurance Group will together offload shares in the automaker, said the people, who declined to be identified because the information has not been made public.

Other financial institutions will also pare back their Honda stakes, bringing the total sale to around 500 billion yen based on Honda's current share price, the sources said.

Honda is set to soon formally give the insurers the go-ahead to sell its shares, the sources said.

The automaker has already announced plans to buy back up to 300 billion yen of its shares during the current financial year, a move seen as helping absorb some of the impact from the sale.

Honda declined to comment on the insurers' sale, saying only that the information was not something it itself had announced.

Spokespeople for Tokio Marine and MS&AD declined to comment. Sompo did not immediately respond to a request for comment.

($1 = 161.5900 yen)

(Reporting by Miho Uranaka; Additional reporting by Maki Shiraki; Writing by Anton Bridge; Editing by David Dolan and Christopher Cushing)

By Miho Uranaka