Google pulled out after signing up and Amazon said it would not get involved.
Microsoft criticised the plan, saying it was given two days to sign up to a "secret" manifesto with no input.
"We had concerns about process and governance that led us to question IBM's intentions," Microsoft's Steve Martin told BBC News.
Cloud computing is the term given to the shift of computing power, storage and usage from the physical machine on people's desks, in homes and in offices to the web.
Companies such as Amazon, Google and Microsoft offer rival cloud computing solutions, including database management, storage, and running programs on remote servers.
The Cloud Computing Interoperability Forum, which had also added its name to the list of more than 30 companies in the plan, withdrew its support over the weekend.
Some of those companies still backing the plan include CISCO, Sun, AT&T, Red Hat, SAP and AMD.
IBM would not comment over the Google change of heart or that of the CCIF but said it hoped Microsoft would reconsider and get involved in the Open Cloud Manifesto at some stage.
"The aim was for this (Manifesto) to serve as a rallying cry to the industry to get focused around the importance of the cloud environment being open," Karla Norsworthy IBM's vice president of Software Standards explained to BBC News.
"We are pleased about the number of vendors who have signed up. As regards Microsoft, we are still hopeful about working together on giving customers the flexibility they have come to expect from technology that is open."
