Carmelo Anthony's 38-year old mistress gave birth to his twins in April.

So did so in London, because Melo suggested making the kids British citizens.  According to Hollywood Unlocked, the mistress -- who was ready to have an abortion but balked when Melo didn't show up to drive her to the clinic -- believes Melo shipped her overseas so he could keep his name off the birth certificate and hide his twins.

The mistress is still in England because the babies were born underweight and are still in the NICU.  The boy twin has had complications and has needed surgery.

She says she's coming forward because she's been trying to contact Melo but he won't respond.

Melo's wife Lala Anthony recently filed for divorce from the Trailblazer.