Swiss bank Julius Baer
The takeover will be officially announced on Monday, one unnamed financial source in London told the Cash website.
Cash also cited sources as saying Baer had offered 2.5 billion Swiss francs (1.7 billion pounds) for Sarasin, which Rabobank had rejected as too low.
Baer was not immediately available for comment on the article but has publically expressed an interest in Sarasin. It is due to publish an interim trading statement on Monday.
Rabobank declined to comment.
Separately, the Finanz und Wirtschaft newspaper cited unnamed sources as saying cooperative retail bank Raiffeisen has made a non-binding offer for Rabobank's stake, but did not plan to delist Sarasin.
A Raiffeisen spokesman declined to comment.
Sarasin, one of a host of smaller private banks in Switzerland, said last month Rabobank was looking at all options for the 46 percent of Sarasin it owns, which corresponds to 68 percent of voting rights.
Its shares rose almost 12 percent this week on takeover speculation after the SonntagsZeitung newspaper reported that Baer was ready to make an all-share offer, rather than a cash and shares deal.
In recent weeks, Swiss media reports have linked both Julius Baer, which analysts estimate has a cash pile of about 800 million francs for deals, as well as Raiffeisen to Sarasin, which traces its origins back to a bank founded in 1841 and employs almost 1,700.
The Swiss private banking industry is seen in dire need of consolidation given rising costs and falling revenues, particularly as the industry has been forced to move away from a business model that often relied on offshore tax evasion.
Baer and Sarasin are the biggest dedicated private banks in German-speaking Switzerland and a takeover would arguably be the biggest shakeup in Swiss banking since the 1998 merger of Swiss Bank Corporation and Union Bank of Switzerland to form UBS AG
($1 = 0.899 Swiss Francs)
(Reporting By Emma Thomasson; editing by Patrick Graham)