Lord Mandelson has admitted that the previous Labour government failed to exploit the close links it forged with Muammar Gaddafi to press him to bring in democratic reforms in Libya.

The close ally of Tony Blair's insisted the former prime minister was right to strike his controversial 2004 agreement to end the Libyan leader's status as an international pariah. But he told The Independent yesterday: "We could and should have done more to promote human rights issues, democratic change and transition in Libya. That is something that you learn."

David Cameron has attacked Labour's "dodgy deals with dictators in the desert" and Mr Blair's links with the Gaddafi regime face fresh scrutiny following its collapse.

An unrepentant Lord Mandelson said he had "no regrets whatsoever" about Mr Blair's decision to offer his "hand of friendship" to Colonel Gaddafi in his Bedouin tent outside Tripoli seven years ago.

He dismissed as "absolute rubbish" claims that the motive was to secure access to Libya's oil and gas reserves for British firms, saying the aim was to halt terrorism and prevent a dangerous dictator developing nuclear, chemical and biological weapons.

He disclosed that, as Business Secretary in 2009, he held talks in London with Saif al-Islam Gaddafi, the Libyan leader's son and favoured successor – a meeting encouraged by the Foreign Office as it was seen as being in Britain's interests.

"He presented himself as the hope for change, a pro-democracy agent. Government policy was to encourage and support change. I don't say Saif was lying but I think he overstated his ability to bring about change and underestimated his father's deep opposition to it. In the end, he decided to stick with his father. I think that was the wrong choice. Now he must take the consequences."

Lord Mandelson said: "We were right at the time to bring Gaddafi in from the cold to deal with his sponsorship of terror and his weapons of mass destruction. That is what it was about."

He claimed the commercial and investment prospects in Libya were not big enough to make them the compelling argument. "You don't reach out to someone one minute and then say, 'We won't trade with you because we don't like you.' If someone is going to become part of the international community, then you don't turn round and say, 'We are not going to trade with you or invest with you.' It is not logical."

Lord Mandelson argued that the current situation in Libya could be much more dangerous if Col Gaddafi had access to WMDs. "If this rebellion had taken place and he was still in possession of his weapons, still able to organise terror, he would have had no hesitation in doing so. We would be in a much worse position if Mr Blair did not do what he did."

The former Cabinet minister accused Mr Cameron of using events in Libya to try to "discredit" Mr Blair but said: "That's just politics. It's neither fair nor justified... Just as it is right to support the rebels now, it was right then to engage Gaddafi in the way that we did."

Lord Mandelson criticised politicians across the spectrum in Britain for not doing enough to help the democracy movement in Syria – and hinted that intervention may eventually be needed there, too.

"Whilst we have all been focused on Libya, we have not given the backing and support to the Syrian people, who have shown extraordinary bravery in standing up to a vicious regime," he said. "There has been a small conspiracy of silence across British politics about Syria. Now it is time for us to follow President Obama's lead and start to get much much tougher with [President] Assad and the regime, even if we cannot repeat at this stage the sort of intervention we made in Libya."

He defended the London School of Economics (LSE) for its close links with the Gaddafi regime, which were fostered by Saif. "Hindsight is a wonderful thing. It is hardly fair to say the LSE or any other institution should have maintained some sort of boycott," he said.