

The creature may have been taller than an ostrich if it had been flightless and, if it flew, had a greater wingspan than that of the albatross, they reported in the British journal Biology Letters.

The scientists have named the bird Samrukia nessovi -after a mythological Kazakh phoenix known as the samruk, and after Lev Nessov, a celebrated Russian palaeontologist who died in 1995.

The estimate is based on a pair of mandibular rami, or the upright part of an L-shaped lower jawbone, that were found in Late Cretaceous sediment in Kyzylorda, southern Kazakhstan.

The bones measure 275mm, indicating a skull that would have been 30cm long.

Whether the bird flew and what it ate are unclear because the evidence is so sketchy.

But if the two bones are a guide, the beast would have stood up to three metres high and weighed more than 50kg if it had been flightless.

If it flew, the bird would have weighed at least 12kg, with a wingspan of at least four metres.

The avian was 'an undisputed giant', says the study.

Birds are believed to have evolved from tiny two-footed dinosaurs called theropods at the start of the Cretaceous era, around 150 million years ago.

The prevailing theory, based on usually-incomplete fossils, is that they remained extremely small for tens of millions of years.

Of more than 100 types of early birds that have come to light, only one - Gargantua philoinos, which lived around 70 million years ago - was large-bodied. The others were crow-sized or smaller.

But even the claim for Gargantua philoinos is under attack, with some scientists arguing that the fossil was really that of a pterosaur, or flying reptile, rather than a bird.