Beattie also said he no longer believes the party reflects a “moderate and progressive outlook”.
He referenced the recent decision by a party councillor Kate Evans, who works in Beattie’s constituency office, to quit the UUP.
Beattie also criticised a move related to another of the party’s nine MLAs, Alan Chambers, who was told not to put his name forward when his constituency association in North Down meets to select its candidate.
He ended his letter by saying he wished the party well and hopes it can “rediscover its social conscience”.
The UUP has been asked for a response.