John Williams: Robin asked me if he was finished. I said he was not

Of all the moments that came back to me when I heard he was dead, two stand out. The first was in May 1998. Robin had had a famously rough first year in the job. His ethical foreign policy had been ridiculed, his marriage had broken up very publicly, and now the Sandline controversy was tearing at his reputation for competence. "Am I finished?" he asked. "No bullshitting." "No," I said. "You can come back from all this."

The second moment was the following summer. He went on a tour of liberated Kosovo, and the crowds chanted: "Robin Cook, Robin Cook, Robin Cook."

Those were the extreme moments of Robin's four years at the Foreign Office, the low point and the high. Nobody should make the mistake of assuming this notoriously prickly, apparently tough performer came through his ordeal of media ridicule by sheer resilience. He hurt badly. Robin's vulnerability - his best kept secret - made his endurance a remarkable achievement.

A lesser figure would have been stripped of self-confidence. But even in that unhappy period Robin maintained a strong sense of moral purpose in the job. It was at this time that he drove along the negotiations which led to the International Criminal Court; negotiated the European Union code of conduct on arms sales; and established the Foreign Office's annual human rights report.

One Saturday morning in 1999, a pile of corpses was discovered on the outskirts of a village in Kosovo called Racak. I rang Robin to discuss a press statement, and found him determined that we had to stop the killing.

Robin Cook, Tony Blair and George Robertson (then Defence Secretary) could have put it in the "too difficult" file. President Bill Clinton had no intention of committing ground troops. Every military commentator said you could not win a war by air power alone. The critics seemed right when the first result of our intervention was a massive refugee crisis resulting from Milosevic's vengeance on the Kosovans. Robin spent the whole day on the phone in pursuit of a haven for the refugees, which he agreed and established with the Macedonian government.

The diplomacy of the Kosovo conflict was largely conducted through daily conference calls between Nato's big five foreign ministers: US, UK, France, Italy, Germany. Robin became the leader of that group, its most creative force, to whom the others looked for solutions. I recall a difficult discussion during which Madeline Albright cut through the babble, saying: "I want to hear Robin's take on this."

The critics of air power seemed vindicated on another Saturday when we were woken by the news that Nato had bombed the Chinese embassy in Belgrade. "I hear we have a problem," said Robin, in that laconic way. He was booked to do the Today programme, which he found stressful in the best of circumstances. He got through it by saying sorry before John Humphrys could demand it.

Robin and George Robertson kept hope alive among the victims of Milosevic's terror. If it was worth Robin being Foreign Secretary for one thing, it would be the thanks of Kosovan leaders. Let's not forget that war was fought in defence of Muslims. It was, in fact, a triumph of ethical foreign policy, but Robin longed to be free of the phrase.

He knew he needed a less grandiloquent way to describe his approach to the world, and he chose "critical engagement". This meant engaging unpleasant regimes, while remaining critical of their excesses, in order to make the world less dangerous.

This led to the breakthrough with Libya that made the Lockerbie trial possible, and to the engagement with Iran which resulted in it no longer supporting the death threat to Salman Rushdie.

Jack Straw has always credited Robin with making clear his objections to the Government's Iraq policy, stating clearly in Cabinet where his bottom line was, and resigning when it was crossed.

I am very confident that Robin Cook will be remembered as one of the best Foreign Secretaries, as well as the finest Commons speaker for a long, long time.

John Williams is director of communications at the Foreign and Commonwealth Office