Mr Thomas enjoyed a convincing win for the Westminster seat taking 10,715 votes - a majority of 4,948.

The 36-year-old replaces Cynog Dafis and becomes the first MP to be elected in the new millennium.

Liberal Democrat Mark Williams (5,768) beat Conservative Paul Davies (4,138) into second place.

Labour's Maria Battle trailed into fourth place with 3,162 votes.

However the new MP said Plaid still had a role to play in London.

"The job is to work in partnership with representatives in National Assembly," he said.

"There is still a great deal of argument to be had at Westminster and there are still a number of points that need to be taken forward at that level."

However he thought the manner of Plaid's win and Labour slump sent a "clear message" to Assembly leader Alun Michael - especially over the issue of European grant aid for Wales.

"This by-election not only sends a message to Westminster, it sends a message to the National Assembly and that message is very clear - that the Labour vote will fall drastically unless Alun Michael comes up with Objective One funding.

"I have been consistently approached on the street by people who say they are very disappointed by the way the current administration is performing and they are unhappy with the leadership.

"Well, Plaid Cymru is ready to give that leadership."

Mr Thomas also suggested that Plaid's threatened no-confidence vote in the Labour leadership over European funding was still on.

"It will happen," he told BBC Radio Wales.

"This is a serious challenge to the Labour party."

"The way the Labour government as abandoned rural areas was a very clear message in the last week of the campaign.

"This victory is a vote of confidence in the future of Wales. We go forward from here," he added.

Results

Under half (45.7%) of constituents visited polling booths compared with 73.9% in the last General Election

While that is higher than some recent by-elections, it will be regarded as low for west Wales which traditionally prides itself on a high level of political awareness.

The results for the seven candidates were:

• Simon Thomas, 36, a rural community development manager (Plaid Cymru - 10,716)
• Maria Battle, 42, a solicitor and director of Age Concern Ceredigion (Labour - 3,612)
• Mark Williams, 33, a teacher, (Liberal Democrat - 5,768)
• Paul Davies, 31, a business banking manager (Conservative - 4,138)
• John Bufton, 37, businessman, (UK independence - 487)
• John Berkeley Davies, 55, a teacher (Independent Green - 289)
• Martin Shipton, 46, a journalist, standing as "Wales on Sunday": Match Funding Now - 55.