The two tracks – blues classic Keys to the Highway, recorded in 1970, and an early version of the famous song Whole Lotta Love – are among dozens of tracks which the band will officially release alongside the reissues.
The companion discs will feature alternative versions of songs, works in progress and live performances, all recorded at the time of the original albums.
The tracks, like hundreds of others, had been carefully catalogued but then locked away in the the band’s archive for decades.
Guitarist Jimmy Page, 70, has spent two-and-a-half years combing through the vaults, listening to hundreds of quarter-inch tapes before choosing the best material.
Page says the new material “deserves to be heard” because “it’s performance art”.
It will give fans the first chance to eavesdrop on some of the band’s key recording sessions.
Keys to the Highway/Trouble in Mind, to appear on Led Zeppelin III, was recorded in 1970 at the Olympic Studios in Barnes in south-west London, about half an hour after Hats Off to (Roy) Harper. But it was never released.
“It was a particularly prolific time,” says singer Robert Plant, “where we were learning about each other’s capabilities.
“Jimmy and I were just fooling around with the stuff that we would play at home. We were just trying things out. Nothing was premeditated.”
Led Zeppelin II features an early version of Whole Lotta Love, which many consider to be one of the greatest rock ‘n’ roll songs of all time.
There are significant differences between the two tracks. The original is missing the first chorus and middle section. New vocals appear on the finished version, as well as a slide guitar.
“You realise just how important all of those additional layers and the filigree work is,” says Page.
“There’s all manner of bells and whistles to make it the song Whole Lotta Love as we all know it.”
Page says it’s “reassuring” revisiting the songs after all this time.
“It’s undeniable that we’re good,” he adds. “The band was the real deal.”
But the inevitable passage of time has given Plant, who is now 65, a different perspective.
“My enthusiasm sometimes got in the way of finesse. I listen to it and go, wow, why didn’t I shut up a bit?” he laughs. “I kind of overcooked it.”
Page promises “lots of surprises” on the three reissues, which are part of a plan to reissue all nine of the band’s studio albums in chronological order.
But he has scotched rumours that there will be versions of songs featuring bass and keyboard player John Paul Jones on vocals.