Leanne Wood AM, Plaid Cymru leader, said: “This very sad news has come as a complete shock.
“Bob has been a tenacious advocate of the rights of members of the RMT.
“I’ve shared a platform with Bob: he has been a good friend to Plaid Cymru, having been a guest speaker at our party conference back in 2003 – the first UK trade union leader to do so.
“Bob’s emphasis on placing the needs of his members above all else - including and especially party politics - will be sorely missed. We need more like Bob Crow.
“He was fearless and would not back down in the face of frequent and unfounded attacks from the ‘right.’
“He was also a firm and vocal opponent of the politics of austerity and was excellent at articulating that there is an alternative to the current, cosy Westminster consensus.
“He will be deeply missed by all on the left throughout the UK.”