There was no substantive evidence, which is why most of the country opposed it. Labour got whipped into following it because of Blair's threat it could bring down the government, and the Conservatives didn't want to appear weak.
They should have turned when Robin Cook (RIP) came out and said in as polite terms as he could muster, that it was all bullshit.
The Lib Dems ranged from requiring multilateral UN agreement in the form of a second resolution (which we never got) to opposing it outright.
Wow! Mr Cook was spot on.
(I think that's how you all say it over there.)