The Welsh band were supporting R.E.M for two dates on their current European tour.
A spokeswoman for Catatonia said Matthews‘ behaviour was probably fuelled by a bottle of wine and had left the more reserved Mills “rather shocked”.
She laughed: “She just ran on, grabbed him and snogged him in the middle of a song. He looked rather taken aback!”
Despite Matthews‘ actions, the spokeswoman said the dates had gone brilliantly and the two bands had got on really well.
Meanwhile, Catatonia release new single ‘Londinium’ on Monday through Blanco y negro, backed by two new tracks, ‘International Sigh’ and ‘Apathy Revolution’.
The band are also currently promoting the 1999 National Year Of Reading in Wales. They are featured on posters on buses with the slogan ‘Get A Book – Get A Life!’.