Northern Irish rocker Tim was invited to sessions for the 'Viva La Vida' band's new album by singer Chris Martin because he is a big fan of the vocal melodies he writes.
A source told The Sun newspaper: "Chris has always had a soft spot for Ash. They were quite close in the early days and have shared a number of bills over the years.
"He admires Tim as a songwriter, especially his brilliant way with melodies, so he didn't think twice about asking him to get involved."
Although it is not known exactly what songs 'Girl From Mars' singer Tim, 34, has had input on for the as-yet-untitled album, Chris has previously said he was considering one to be a duet.
He said: "We've narrowed it down to 15 tracks. I'm trying to make sure there are three songs that start with the same letter. On the last album it was 'L'. It might be 'U' this time.
"One of them is 'Up With The Birds'. It might be a duet, but we don't know who with yet. The other two might change."
Chris, 33, has also revealed track titles slated for inclusion on the record include 'Hurts Like Heaven' and 'Every Teardrop Is A Waterfall', which will likely be the last track.
The album is expected to be released later this year.