The consortium, made up of investors Arcus and Peel Group and RREEF, an arm of Deutsche Bank, is expected to walk away.
A spokesman for Forth Ports would not confirm the group would ask the Takeover Panel to intervene.
He said: “We have got a business to run for the benefit of all shareholders.”
Nobody at the Northstream consortium was immediately available for comment.
Forth Ports is currently valued at £618m.