Rudolph Isley, whose smooth vocals graced the Isley Brothers' hits including Summer Breeze and That Lady, has died at the age of 84.

The musician, who also co-wrote many of the band's biggest songs, died in his sleep, his brother Ernie said.

"There are no words to express my feelings and the love I have for my brother," added Ronald Isley in a statement.

"Our family will miss him. But I know he's in a better place."

Although Rudolph largely sang harmonies with the band, he took lead vocals on such tracks as I've Got to Get Myself Together and It's a Disco Night (Rock Don't Stop), which reached the Top 20 in the UK.

He also played a pivotal role in writing songs such as Harvest For The World, Fight The Power and Shout - an enduring party anthem that became a major hit in Europe through Lulu's cover version.

First formed in the early 1950s, the Isley Brothers were among the most influential bands in pop music, successfully shifting from gospel to Motown soul, and later gritty R&B and politically-motivated funk.

Hailing from Cincinnati, Ohio, the young group were initially a quartet comprised of Ronald, Rudolph, O'Kelly and Vernon Isley.

They briefly stopped performing in 1955 after Vernon, who sang lead vocals, was killed while riding his bicycle. He was just 13 years old.

Eventually persuaded to continue, the others moved to New York and left gospel music behind.

On tour in 1959, they covered Jackie Wilson's Lonely Teardrops - drawing out the closing call-and-response section and whipping the crowd into a frenzy.

By the end of the tour, "audiences were coming to the theatre and waiting for the song", Ronald later recalled.

They quickly began developing the outro into a song of their own and, when they returned to New York, went straight to the studio to cut it, bringing in dozens of friends to capture the energy of their concerts.

That song was Shout, which became the Isley Brothers' first million-selling record. It was inducted into the Grammy Hall of Fame in 1999, but at the time church groups objected to it.

"We turned a song with a gospel feel into an R&B hit, and the groups began writing [to] disc jockeys asking them to stop playing our record," Ronald told the Wall Street Journal in 2015.

"They felt Shout should have been a church record."

Despite the criticism, the brothers knew they'd found a successful formula, and scored another hit in the early 60s with the equally-spirited Twist and Shout.

The song was a cover, originally recorded by the Top Notes, but the band's new arrangement was like a shot of adrenaline to the heart. The Beatles loved it so much, they made it a staple of their lives shows and recorded it the closing track of their debut album.

In 1964, the Brothers recruited a young Jimi Hendrix to their line-up, with his guitar work livening up hits like Testify before he left to go solo.

A year later, the band signed to Motown Records, but found the label's production line approach stifling.

During their brief spell there, they only scored one hit, This Old Heart of Mine (Is Weak for You), written by the celebrated Holland-Dozier-Holland team with input from Sylvia Moy.

Leaving to form their own label, they welcomed younger brother Ernie to the band, and his hard-edged guitar lines gave them a new lease of life.

In 1969, they scored a major hit with It's Your Thing and started covering rock anthems like Stephen Stills' Love The One You're With and Bob Dylan's Lay Lady Lay.

That triggered a dramatic rebirth, with a run of gold and platinum albums that fused soul melodies with spacey psychedelia and the hard-edged funk of Sly Stone and James Brown.

The transformation was made explicit on 1973's 3+3 album, where they retooled the jaunty 1964 single Who's That Lady with a searing fuzztone guitar line, taking the song to number six in the US charts.

More hits followed - Summer Breeze, Harvest For The World, Fight The Power - all of which Rudolph co-wrote.

On record sleeves, Rudolph stood out for his flamboyant outfits - a combination of hats and furs accentuated by a bejewelled cane.

The second-eldest son of the Isley clan, he was nonetheless the unspoken "overseer" of the band, daughter Elizabeth Isley Barkley wrote in her memoir.

But, as with all families, relationships were often complicated.

"At times, it got pretty heated within the family, and they'd be yelling at each other," recalled engineer John Holbrook, who worked on several Isley Brothers albums in the late 70s.

"They were big, scary guys. Rudy was the giant and kind of intimidating."

After O'Kelly died of a heart attack in 1986, Rudolph decided to follow his long-held ambition to become a Christian minister, leaving the band and passing the cane on to brother Ronald.

"Music and faith, they just run through our blood," Rudolph said in the liner notes for a 1999 Isley Brothers box set.

"I may have stopped singing pop music, but I will always be an Isley Brother."

He continued to sing, however, releasing a religious album called Shouting for Jesus in 1996, and was inducted to the Rock 'n' Roll Hall of Fame with his brothers in 1992.

This March, Rudolph sued Ronald, claiming his brother had tried to secure a trademark for the Isley Brothers' under his own name, effectively excluding his sibling from the partnership.

The lawsuit claimed that the founding members were "at all times" a "common-law partnership".

His death leaves Ronald and Ernie as the only remaining brothers from the band.