A 77-million-year-old dinosaur skeleton is expected to fetch up to $US8 million ($A12 million) when it goes up for auction in the United States this month.

The complete fossilised skeleton of a Gorgosaurus, which stood more than three metres high and six metres long, is to be put up for sale by Sotheby’s in New York on July 28.

This carnivore of the tyrannosaurid family lived in western North America during the late cretaceous period and was found in 2018 in Montana. 

It is the only example of the species available to private collectors, with all other specimens housed in museums and other institutions.

The Gorgosaurus was a close relative of the well-known Tyrannosaurus rex – although the latter lived some 10 million years later – and, although it was somewhat smaller than its more famous cousin, the two ancient meat-eaters shared many characteristics, including large heads, a mouthful of curved and serrated teeth and small two-fingered forearms.

A male adult weighed up to two tonnes and some palaeontologists speculate the Gorgosaurus was faster, even more ferocious and had stronger jaws than T. rex, according to Sotheby’s.

The specimen to be auctioned was, at the time of its death, a large adult and its cranium was especially well preserved, according to the auction house, which emphasised the “immaculate” condition of the skeleton thanks to the characteristics of the ecosystem where the fossil was found.

The skeleton will be displayed to the public from July 21 at Sotheby’s New York gallery.

The auction house has a long history of selling dinosaur fossils and in 1997 was the first to offer a complete skeleton, the famous T. rex known as Sue, which was sold for $US8.36 million ($A12.3 million) and can be seen at Chicago’s Field Museum.