The actress, who lives in rural Montgomeryshire, is currently in the United States from where she telephoned the "Yes For Wales" office in Cardiff: "I speak as someone who is not Welsh but lives in Wales. I believe in decentralisation and therefore I favour a Welsh assembly."
The endorsement was announced at a party in Hay Castle, once a stronghold of Henry IV during his dispute with the Welsh patriot Owain Glyndwr over the right of Wales to govern itself. Nearly 600 years on, the castle, the domain of Richard Booth, the "King Of Hay", has switched sides to become a bastion of the pro-assembly movement.
Mr Booth and Eluned Morgan, Labour MEP for Mid- and West Wales, hosted the event, which attracted figures from Wales's political and arts establishments.
Speeches preceded performances for harp, flute and soprano by Sherazade, a trio of women who travelled from London for the party in the castle's newly-refurbished State Room.
Political chords were struck by Ms Morgan, a leading figure in the "Yes" campaign, and Liberal Democrat MPs Richard Livsey (Brecon and Radnorshire) and Lembit Opik (Montgomeryshire) who claimed that the tide for a "Yes" vote in the 18 September referendum was rising.
"The breadth of support is widening all the time," Ms Morgan said. Hay and other Welsh communities needed to look to fresh champions in order for their voices to be heard more strongly both in the UK and the European Union.