Michael Douglas' Ex-Wife -- 'I Stand By Michael -- Let Me See My Son!'

Michael Douglas and his ex-wife -- who once battled bitterly -- are in lockstep over what they call the barbaric treatment of their imprisoned son ... TMZ has learned.

Diandra De Morrell Douglas tells TMZ ... their son Cameron has been treated like an animal while in solitary confinement at the Cumberland Federal Prison in Maryland. He's serving 10 years for heroin possession and conspiracy to distribute narcotics. Cameron failed a drug test in prison and was placed in solitary until recently.

As you know ... Michael talked about Cameron after winning an Emmy for "Behind the Candelabra," saying he wants to visit his son badly but isn't allowed.

Diandra tells us when Cameron got out of solitary he couldn't walk in the daylight, because his skin would burn because it couldn't adapt after living in an underground hole for so long. His hair is also falling out.

Diandra says even though Cameron is out of solitary, part of his punishment is no visitors.