James Hetfield believes Metallica's 'Black Album' was the "master key" to their popularity and success.

The 58-year-old singer admits the self-titled record led to the heavy metal outfit becoming a "force to be reckoned with" in the music industry.

The band released 'Metallica' - which has a black cover, compete with the band's logo and a coiled snake - 30 years ago in August 1991, and it has become known by fans and the group as 'The Black Album'.

James said: "The 'Black Album' really was the master key to everything.

"We started to be recognised and talked about as a force to be reckoned with in the heavy metal world - and going beyond that, in the rock world, with bands like AC/DC and U2.

"We were super-proud to be a part of that legacy and to be able to take Metallica to the next level.

"And what made it all better was that the mainstream came to us.

"It was to be, you know, in the grocery store and someone's mom would say, 'Oh, my kids really love your music ... and so do I.'

"So what the 'Black Album' did was ... well, it made us really popular, basically."

James believes the record - the band's fifth studio album, following on from the likes of 1983 debut 'Kill 'Em All', and 1986's 'Master of Puppets' - gave Metallica the licence to "go where we wanted to go".

He added to Guitar World magazine: "It gave us carte blanche to be whatever we wanted to be, and to go wherever we wanted to go.

"So we're very aware of what 'The Black Album' was, what it did and the doors it opened for us.

"And now we're showing our respect for it."

Metallica are doing so by reissuing and remastering their 1991 album, which has sold 35 million copies worldwide.