Prayer in Reform and Conservative Shuls

Dear Rav, Recently, due to family obligations I was forced to attend an engagement party in a conservative shul. The party was hed in a social hall, but during the party, many ’Orthodox’ people decided to Daven Mincha in the main Sanctuary. I had always been under the impression that it is forbidden to even step foot into a conservative, or reform sanctuary, because it stands for values contrary to those of religous Judaism. What is the definitive Halacha on this?

Rav Moshe Feinstein (Shu"t Iggrot Moshe Orach Chayyim IV 91) prohibits davening in a Conservative synagogue even in a separate room that is not used for Conservative services. This because of Conservative rejection of basic principles of faith and observance.