Plaid Cymru's Westminster leader Elfyn Llwyd MP called on Saturday for the keys to Welsh prisons to be handed to the Welsh National Assembly.
On the final day of his party's conference in Brecon Mr Llwyd said last week's rightwards reshuffle by Prime Minister David Cameron had "reinforced" Plaid's view that powers over criminal justice should be passed to Wales.
The Tory PM brought in Chris Grayling to replace Ken Clarke as Justice Secretary.
And Mr Llwyd said: "Now we are left with a secretary who will speed up the privatisation of the prison estate and youth justice - a catastrophe waiting to happen, if the mess that surrounded G4S before the Olympics was anything to go by.
"Policing powers should lie in the hands of the Welsh government."