Phil Williams spent much of his adult life campaigning for Plaid

His sudden death at the age of 64 was announced on Wednesday night.

Dr Williams had retired from the Welsh assembly only six weeks ago to return to his first career as a space scientist.

Plaid leader Ieuan Wyn Jones described him as a genius, and Conservative assembly member Alun Cairns called him one of the kindest and most generous people he had known.

Former Plaid president Dafydd Wigley said: "This is very sad news.

"He had to correct so-called experts that came to the assembly" Conservative Alun Cairns

"It is a huge loss to Wales and to the scientific world. He was a man who still had a major contribution to make."

Police said they were investigating the death of a man in Cathays, Cardiff. But a police spokesman said it was not being treated as suspicious.

Dr Williams was an expert in both economics and astronomy

Dr Williams had been Plaid's economic spokesman in the assembly for its first four-year term, from 1999 until he stepped down last month.

In a statement Plaid said: "Phil spent his life fighting for the people of Wales and was a huge influence on Plaid Cymru.

"Everyone who met him was well aware of his formidable intellect and abilities.

"His contribution to the fledgling assembly was immense.

Phil Williams' life and career 1939: Born Tredegar 1967: Fellow, Clare College, Cambridge 1967: Physics lecturer, Aberystwyth 1968: Plaid Caerphilly by-election candidate 1970-76: Plaid chairman 1991: Physics professor 1999 - 2003: Plaid AM

"We extend our sympathy and deepest condolences to his family. He will be missed by all those within the party who have lost a friend and mentor."

Ieuan Wyn Jones, who will step down later this year as Plaid leader, said: "I have known Phil Williams for over 30 years and always knew that I was in the presence of a genius."

Plaid MEP Eurig Wyn, speaking from Brussels, said: "His knowledge and capacity to understand and disseminate the details of European affairs was immense and will be sorely missed in Plaid Cymru by elected representatives so dependent on his expert advice.

He came to prominence at a 1968 by-election in Caerphilly

"I convey my condolences to all his family."

Dr Williams, who had been AM for south-east Wales, was also physics professor at the University of Wales, Aberystwyth.

Tory AM Alun Cairns, who sat with Dr Williams on the assembly economic development committee, said: "There's no doubt that Phil was one of the kindest and most generous persons I have ever met.

"I consider it a privilege to have known him and to have worked with him.

"His commitment to devolution was second to none. He had one of the best brains I had ever come across and was prepared to work with any party to improve the quality of life.

Phil Wiilliams leaves a widow and two children

"I can recall a number of occasions where he had to correct so-called experts that came to the assembly's committee. Always polite and courteous and almost embarrassed by his intellect.

"He is an enormous loss to Wales and was one of the world's leading authority in his field. Few have achieved so much in their life as Phil."

Plaid chairman John Dixon said Dr Williams was a leading light in the party and had a distinguished history.

"His expertise and economic acumen were second to none," said Mr Dixon.