European leaders must set out the consequences Moscow will face for the annexation of Crimea - including further action on travel bans and asset freezes, at a European Council today, the Prime Minister said.
Fears are growing of a major escalation in violence in Crimea after two people died in clashes between Ukrainian and pro- Russian forces.
Mr Cameron said: "If we turn away from this crisis and don't act we will pay a very high price in the longer term."
He told the Commons the Crimean referendum had been "spatchcocked" together in the space of 10 days "at the point of a Russian Kalashnikov".
The Prime Minister added: "I also think we should be responding to the fact of this annexation. We said that if there was further action to destabilise the Ukraine, and this annexation is that action, further consequences need to follow.
"We need to set that out on Thursday, in concert with our European partners and at the same time I think we need to put down a very clear warning that if there was further destabilisation - for instance going into the eastern Ukraine in any way - then we would move to a position of the sorts of economic sanctions that we discussed last week."
Labour leader Ed Miliband said his party would back "the toughest possible" economic and diplomatic measures against the Russian Federation.
Britain supports a call from US president Barack Obama for leaders of the G7 nations - the US, UK, Germany, France, Italy, Japan and Canada - and the EU to meet to discuss the crisis on the fringes of a nuclear security summit in the Netherlands next Monday.
Mr Cameron said: "I think it is important we move together with our allies and partners and I think we should be discussing whether or not to expel Russia permanently from the G8 if further steps are taken."
The Office for Budget Responsibility (OBR) warned of the potential harm the Ukraine crisis could do to the UK economy if the situation deteriorated.
It said: "Developments in Ukraine are not expected to have a large impact on the UK in our central forecast. However, if the situation escalates or continues for a prolonged period, there is a risk of higher commodity prices affecting inflation and output growth.
"There could also be a broader risk through trade linkages and financial exposure to Ukraine, Russia and other affected countries."
The Treasury's Budget Red Book said the crisis was a "significant new risk" to the economic recovery and "any further deterioration is likely to have some impact on the UK".
The Prime Minister discussed events in the Ukraine with German Chancellor Angela Merkel ahead of the summit.
A Number 10 spokesman said: "They reiterated that Sunday's referendum in Crimea and Russia's subsequent actions to annex Crimea are both illegal and agreed that the EU should impose further consequences on Russia, building on the travel bans and asset freezes agreed by European foreign ministers on Monday.
"They also agreed that the EU should keeping working with international partners to de-escalate the situation.
"A strong and successful Ukraine is in all our interests and efforts to undermine the country's stability will not succeed."