Guinness will meet with London Irish to discuss concerns over the signing of Paddy Jackson ahead of next season.
Guinness and London Irish have been partners for more than 30 years but Diageo, the parent drinks company, said it had " serious concerns " over the signing of Paddy Jackson.
The 27-year-old, who was acquitted of rape last year, signed for the newly promoted Premiership club last month after one season with French side Perpignan in the Top 14.
Jackson is set to reunite with Declan Kidney and Les Kiss at London Irish. It was Kidney who awarded Jackson the first of his 25 Ireland caps in 2013, while Kiss has also coached the fly-half at both Ulster and Ireland.
Kidney said last month: "Players of the calibre of Paddy Jackson do not become available very often and both myself and Les are looking forward to working with him in the future."
Diageo said in a statement released on Monday: "We are meeting the club this week to discuss our serious concerns regarding their decision, which is not consistent with our values."
The Diageo statement follows the news that former sponsor Cash Converters discontinued its relationship with the club in recent weeks.
Cash Converters said in a social media post: "As a company, we are committed to the highest possible standards when it comes to our investments in any sponsorships and collaborations. As a result of a detailed and thorough review of our support for London Irish, we have decided to discontinue our association with the club."
However, London Irish denied that Cash Converters' decision was specifically related to the signing of Jackson.
A statement from the club read: "The club would like to clarify that the decision to part ways at the end of 2018/19 season was made before the announcement of player signings in May 2019, and for reasons unrelated to player signings."