Scientists have discovered the two biggest black holes ever observed, each with a mass billions of times greater than the Sun's, according to a study published Monday.

The two giants are located in the heart of a pair of galaxies several hundred million light years from Earth, said the study in scientific journal Nature.

Each black hole is estimated to have a mass about 10 billion times greater than the sun, dwarfing the previously largest-known black hole, which has a mass of 6.3 billion suns.

The University of California, Berkeley, team led by Nicholas McConnell and Chung-Pei Ma said one black hole is located in NGC 3842, the brightest of a cluster of galaxies about 320 million light years from Earth.

The second hole is of "comparable or greater mass" and is located in NGC 4889, the brightest galaxy in the Coma cluster, about 335 million light years away.