SHANGHAI/BEIJING (Reuters) -The Beijing Stock Exchange has de facto implemented a new policy that prevents major shareholders of companies listed on its bourse from selling stock, worried that such sales could douse a market rally, three people familiar with the matter said.

A "major shareholder" is one with a stake of 5% or more, and to sell shares in a company they own, they need to make a public filing with the relevant stock exchange, according to rules for the country's bourses.

The Beijing exchange has been rejecting those filings, said the people who were not authorised to speak to media and declined to be identified.

It was not immediately clear how long this new policy would remain in place, they added.

The bourse and the China Securities Regulatory Commission did not immediately reply to requests for comment.

The so-called window guidance - where directions are made orally without written documents - could help sustain the upward momentum for the Beijing Stock Exchange 50 Index.

It jumped some 10% on Monday morning following a 21% surge last week that comes after government efforts to revive the long-dormant Beijing market.

(Reporting by Beijing and Shanghai newsroom; Editing by Edwina Gibbs)