New member: David Trimble with Tory leader David Cameron

The former NI first minister, who led the Ulster Unionists for 10 years from 1995, said the move was part of a "long-term ambition".

He told BBC Radio Ulster that it was something that had been on his mind many times over the years.

"Now that Northern Ireland is settled, I'm free to follow what's been an ambition for many years," said Lord Trimble - who became a peer last June.

"I took this decision several months ago and held back the implementation of it so that nothing could happen in the run up to the election," he added.

"I am delighted to see the party come through a very serious challenge in which a very serious attempt was made to destroy it and I look forward to the party re-building."

The Nobel Laureate said the move would give his political career a "wider horizon" but revealed that he did not want to "rush into" taking on a shadow cabinet post.

I'm delighted to see the party come through a very serious challenge in which a very serious attempt was made to destroy it and I look forward to the party re-building Lord Trimble

Tory leader David Cameron said it was not every day that you could welcome a Nobel Peace Prize winner into your party.

Speaking outside Westminster, he said: "I am sure David Trimble is going to bring a huge amount to our party in the House of Lords - not just on the subject of Northern Ireland - but more broadly on questions of security and terrorism and our constitution and developing the Conservative Party for the future."

Unionist movement

Lord Trimble won a Nobel Peace Prize, along with the SDLP's John Hume, for his part in the 1998 Good Friday Agreement.

However, he stepped down as leader of the Ulster Unionist Party, after losing his Westminster seat in Upper Bann in the 2005 general election.

In total, the Ulster Unionist Party lost four of its seats in that poll.

Lord Trimble said the move had been a long-term ambition

It is thought the links between the unionist movement and the Conservatives make it a natural transfer for Lord Trimble.

Sir Reg Empey, who succeeded Lord Trimble as Ulster Unionist leader, paid tribute to him.

"I think this is a smart move. In the event of David Cameron winning an election it would put somebody who really has the interest of the Union at heart right in the centre of national political life," said Sir Reg.

"Through his vision and commitment, David, along with others, helped to transform politics here in Northern Ireland."

UUP peer Lord Rogan of lower Iveagh said Lord Trimble's decision to become a Conservative peer was understandable and natural.

"There has been a long, historic tradition of Ulster Unionist figures at Westminster co-operating with the Conservative Party," Lord Rogan said.

During a visit to Belfast last October, David Cameron held the door open to Lord Trimble joining his party.

While the Tory leader denied there had been any discussion at that time, he said: "David Trimble is a friend of the Conservative Party."