Plans for a G8 meeting in Russia have been on hold since tensions rose following the movement of Russian troops into Crimea.
And Mr Cameron told the Commons yesterday: "If Russia takes further steps, then clearly one of the measures we could bring forward relatively quickly would be a different approach of going back to a G7 rather than a G8. But let's hope that isn't necessary."
Mr Cameron condemned plans to hold a referendum in Crimea to formalise Russia's control over the province.
He told MPs: "We are all clear any referendum vote in Crimea this week will be illegal, illegitimate and will not be recognised by the international community.
"As I discussed with (German) Chancellor Merkelr, Russia can choose the path of de-escalation by signalling it understands the outcome cannot be acted on as legitimate."
The Prime Minister said he had acknowledged steps to tackle Russia's aggression could lead to damage in Britain and elsewhere in Europe.
But he added: "The costs of not standing up to aggression are far greater."
Mr Cameron said Russia could still resolve the crisis diplomatically if it chose to do so and outlined several key steps he felt were necessary.
He said these included direct talks with the Ukrainians, the return of Russian troops to their bases and the withdrawal of support from the pending referendum.
Mr Cameron added: "Ukraine should be able to choose its own future.
"We must stand up to aggression, uphold international law, and support the Ukrainian government."
In Crimea yesterday, unidentified armed men fired in the air as they moved into a Ukrainian naval base near Bakhchisaray.
Ukrainian defence official Vladislav Seleznyov said the group of ten men demanded trucks but no-one was hurt in the shooting.