The party confirmed that Mr Opik, a former leader of the Welsh Lib Dems, is no longer a member of the party.
But Mr Opik says that he had a right to appeal through a formal process.
The controversy is believed to centre around a political meeting that the former Mid Wales MP is said to have attended.
He has been a member of the Lib Dems for 30 years and was Montgomery MP for 13.
Mr Opik has demanded that the party address what he calls a breach of his rights.
"I understand that an individual has complained about me in the party but I have the right to appeal against these complaints through a formal process," he said.
"The party is disrespecting its own processes.
"This is a massive breach of my rights. False accusations have been made against me yet it seems the party has declared the outcome of its processes before those processes have finished.
"I don't want to go through the details of the case as that would make me as bad as the people who have violated the process.
"However this current breach of protocol bears no resemblance to the system of justice within the party that I helped to create.
"After all the time I have invested in the Liberal Democrats over the past three decades I am extremely upset by the mishandling of what should be a comparatively simple complaints procedure. This is an internal process that has not run its course.
"It is a very public breach of due process."