Tony Blair is set to offer British military training for Libyan troops when he meets Colonel Muammar Gaddafi.
It was announced last month that Blair would meet Colonel Gaddafi
The prime minister is defending his visit, saying he was offering "our hand in partnership" to states giving up terror and banned weapons.
As the Libya trip was confirmed, the Tories claimed it would distress families of Lockerbie bombing victims.
Tory Michael Ancram described the trip's timing - shortly after a Madrid memorial service - as "astonishing".
The Lockerbie bombing killed 270 people and their families are divided on the wisdom of the Libya talks.
Mr Blair's meeting comes after Libya announced in December that it was ending its weapons of mass destruction programmes.
The offer of military help could involve Libyan officers coming to British training academy Sandhurst.
BBC News political editor Andrew Marr says the idea is to help persuade Libya it does not need weapons of mass destruction to defend itself.
Earlier, Mr Blair attended the state memorial service for the Madrid bomb victims.
He also held talks with Portuguese Prime Minister Jose Manuel Durao Barraso in Lisbon.
Speaking afterwards Mr Blair said the alternative to beating al-Qaeda was to live in "their shadow of fear".
Defending his Libyan trip, Mr Blair added: "Let us offer to states that want to renounce terrorism and the development of weapons of mass destruction our hand
in partnership to achieve it, as Libya has rightly and courageously decided to
"That does not mean forgetting the pain of the past but it does mean recognising it's time to move on."
Swire, whose daughter died at Lockerbie, criticised Tory remarks
He added: "At this time, with the tears and tragedy of Madrid in our minds, let us unite, put aside our differences, refuse to be divided by terrorism and express our strength and confidence in our own way of life which we will defend to the end."
Andrew Marr told BBC News Mr Blair was not taking any "big geopolitical risk" as Libya is unlikely to take up international terrorism again.
However, Col Gaddafi "is an extremely unpredictable character, who says very, very odd things from time to time," Andrew Marr added.
At prime minister's questions deputy Tory leader Mr Ancram told Deputy Prime Minister John Prescott, that Mr Blair's visit was "highly questionable" and its timing even more so.
Britain had suffered from Libyan support for terrorism through Lockerbie, the murder of Wpc Yvonne Fletcher and backing for the IRA, he said.
Earlier, Tory leader Michael Howard questioned the visit, telling BBC Radio 4's Today programme: "I imagine it will cause considerable distress to the families of the victims of Lockerbie."
Liberal Democrat peer Lord Steel accused Mr Howard of "crazed opportunism" over the comments.
And Jim Swire, from the UK Families Flight 103 campaign group, told BBC News 24 it would have been a good idea if Mr Howard had talked to the families before making his comments and that as far he knew he had made no effort to do so.
Mr Swire said Mr Blair's visit was the next step in welcoming Libya back into the community of nations.
Foreign Secretary Jack Straw has said that no progress would be made on issues such as investigating Wpc Fletcher's death without engagement with Libya.
The police officer is thought to have been shot dead by a gunman inside the Libyan embassy as she helped police a demonstration outside in 1984.
In 1999, Libya accepted "general responsibility" for the killing and agreed to pay compensation to her family.