Chelsea have as yet failed to comment of the reason but Boro boss Gareth Southgate has said: "It's a great shame for everyone involved that Robert won't be joining us at the present time.
"But I'm continuing to pursue a number of avenues towards new signings."
Huth who is suffering from an ankle injury which curtailed his World Cup had a medical on Monday.
On Wednesday, noises from Boro suggested that Huth would no longer be joining them on a £5m deal.
Asier del Horno is a target for Valencia.
The Spanish club have comfirmed their interest in the left-back.
Club president Juan Sole told Las Provincias newspaper: "Del Horno is a very interesting player and our technical team has been following him".