The rest of the band members started having rehearsals for stand-in singers, and when they came across an American, who was very impressive, they were confident that they could go ahead without the rock singer. When Plant saw that the group was serious about going ahead with the planned tour without him, he was finally persuaded to join them.
"The rest of the band had all but given up on Robert joining them, but they were determined to go ahead so had started to seriously explore other avenues," the Daily Telegraph quoted a source as saying. "When Robert realised the band were serious about doing it without him, it made him think long and hard.
"He realised he couldn't face the thought of not being involved. The band were over the moon when he told them the news. They are now forging ahead with the tour plans and they can"t wait to get on the road," the source added.
The idea for the tour, which is set for next summer, came after their successful one-off reunion show at London's O2 Arena in December. Led Zeppelin, who have sold more than 300million albums worldwide, formed in 1968 and split up in 1980 after drummer John Bonham, Jason's father, died aged 32.
Rolling Stone magazine dubbed them the biggest band of the 70s.