Southern Speed is out of Tuesday's Melbourne Cup after suffering a career-threatening leg injury at trackwork.

Trainer Leon Macdonald said the mare had worked well on Saturday morning but jockey Nash Rawiller had concerns when he was pulling her up.

"Nash said he wasn't happy with how she pulled up and when I went to look at her it was obvious she had a bad injury to a fore leg," Macdonald said.

"I'm not sure what happens now. She will be assessed over the next few days."

The 2011 Caulfield Cup winner, Southern Speed missed that race this year and ran an eye-catching sixth in the Cox Plate to set up a Melbourne Cup start.