LISBON/TRIPOLI (Reuters) - British Prime Minister Tony Blair, offering a "hand in partnership," flew to Libya on Thursday for a historic meeting with Libyan leader Muammar Gaddafi, once reviled as a pariah in the West.

Blair has promised not to "forget the pain of the past" after the Lockerbie airliner bombing which killed 270 people and accusations that Gaddafi armed Irish Republican Army guerrillas.

But Blair believes Gaddafi should be welcomed into the international fold for turning his back on weapons of mass destruction.

"Let us offer to states that want to renounce terrorism and the development of nuclear, chemical or biological weapons our hand in partnership to achieve it as Libya has rightly and courageously decided to do," Blair told a Lisbon press conference before his landmark visit to Tripoli.

"That does not mean forgetting the pain of the past but it does mean recognizing change when it happens," he said.

Libya said in December it would abandon efforts to acquire weapons of mass destruction in a bid to mend ties with the West after agreeing to pay damages for the 1988 PanAm plane bombing over Lockerbie in Scotland.

Gaddafi, once condemned by former U.S. President Ronald Reagan as "this mad dog of the Middle East," will meet Blair in a tent outside Tripoli. He is the first British leader to visit Libya since Winston Churchill in World War II.

Critics accuse Blair, who is following in the footsteps of the Italian and Spanish leaders in visiting Tripoli, of being motivated by trade prospects rather than politics in what they see as an act of misguided appeasement.

Britain's opposition Conservatives said the timing of the visit was "highly questionable" coming a day after he attended the memorial service for the Madrid train bombing victims. That was Europe's biggest terror attack since the Lockerbie bombing. But many relatives of the Lockerbie victims were supportive of the diplomatic milestone.

Jim Swire, a prominent spokesman for some British victims, told Reuters: "So long as Libya is indeed turning her back on terrorism we feel this is a good thing.

"We regard the reintegration of Libya into the community of nations as a good move."

British firms are certainly keen to exploit business opportunities in Libya now that Gaddafi is now coming in from the cold.

The Anglo-Dutch oil company Royal Dutch/Shell could sign an outline deal within days for offshore gas exploration rights while defense contractor BAE Systems has announced it was in talks on aviation projects, including potential aircraft sales.