But while his schedule remained unaltered, the meeting itself will be utterly different to the one for which he had been preparing.
As so often occurs in international diplomacy, official agendas get blown off course by events – but rarely in such appalling circumstances and so comprehensively.
The G20 discussion – on economic matters, Syria, migration and the environment – will be totally overshadowed by events in Paris, immediate security concerns and the fight against terrorism. The French president, François Hollande, will not be there and will be replaced by his foreign minister, Laurent Fabius.
Cameron drove up early on Saturday from Chequers to Downing Street to chair the emergency meeting of Cobra, no doubt fully aware that there will be huge implications for his domestic and foreign policy over coming weeks, months and years.
The first chance for parliament to discuss the fallout will come on Tuesday, when the prime minister is expected to make a statement to parliament on both the G20 meeting and the Paris atrocities.
In the short term the government, as Cameron suggested on Saturday, will want to provide more resources for security, intelligence services and policing.
All Whitehall departments will be mobilised behind a national anti-terror effort. Expect George Osborne to be signing off more money in his autumn statement on 25 November to keep the British people safe.
For Cameron’s relations with other EU leaders, and with the EU itself, the Paris attacks will have profound implications. On Saturday, the prime minister stressed that the UK would be working with France in every way it can. “Your pain is our pain. Your fight is our fight,” he said. Working together would be crucial. As was the case during the Greek economic crisis earlier this year, which posed an existential threat to the euro, Cameron’s efforts to drag European leaders round a negotiating table to tinker with the EU treaties, in order to appease his Eurosceptics, will suddenly all seem rather parochial, rather trivial. President Hollande or the German chancellor, Angela Merkel, will not be focusing too much on all that for some time to come.
That Cameron should have tabled his pre-referendum demands for EU reform in the same week as the Paris disaster highlights the narrowness of the cause.
Debate in the EU will now turn, by contrast, to the massive issues of borders and the free movement of people. All the more so following the revelation that one of the perpetrators of the Paris atrocities arrived in Europe via Greece as an apparent refugee.
The migration crisis has already cast into doubt the future of the Schengen open borders agreement as member states, including Slovenia, Hungary and Sweden, have breached it to stop more people pouring in. If Schengen collapses, so does free movement of people and goods, and potentially – as Jean Asselborn, Luxembourg’s foreign minister, said last week – the EU itself.
On Saturday, in a sign of things to come, Poland’s new rightwing government said it would not take part any more in the EU plan to relocate 160,000 refugees around member states. “After the tragic events of Paris we do not see the political possibility of respecting them,” said Poland’s European affairs minister, Konrad Szymanski.
That other European leaders will have more important issues on their minds than the UK’s desire to scratch a largely meaningless commitment to “ever closer union” out of the Treaty of Rome might seem to be bad news for Cameron. He is less likely to get the changes he wants. But on the other hand, the combination of migration and terror may trigger real change in the EU on the very issue that the prime minister needs movement on: free movement across borders and immigration.
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