Mr Miliband spent time in Georgia last week
Foreign Secretary David Miliband has said Russian recognition of Georgia's breakaway regions is "unjustifiable and unacceptable".
Russian President Dmitry Medvedev has declared he formally recognises South Ossetia and Abkhazia's independence.
But Mr Miliband urged Russia to "abide by international law as the basis for resolving this crisis".
Moscow's move has been criticised by the US, France and Germany. Nato says it violates UN resolutions.
Earlier this month Russia and Georgia fought a brief war over the provinces, which already had de facto independence.
Mr Miliband said Moscow must implement "urgently and in full" its promise to withdraw troops to positions they held before the confrontation.
"Today's announcement by President Medvedev that Russia will recognise South Ossetia and Abkhazia is unjustifiable and unacceptable," he said.
"It will also not work. It is contrary to the principles of the peace agreement, which Russia recently agreed, and to recent Russian statements.
"It takes no account of the views of the hundreds of thousands of Georgians and others who have been forced to abandon their homes in the two territories."
He said the move "further inflames an already tense situation" adding: "We fully support Georgia's independence and territorial integrity, which cannot be changed by decree from Moscow."
The foreign secretary said he would visit Ukraine on Wednesday to assemble the "widest possible coalition against Russian aggression".
Earlier, on announcing that Russia recognised the independence of the two states, Mr Medvedev said: "That was no easy choice to make, but it is the sole chance of saving people's lives."
He blamed Georgia for failing to negotiate a peaceful settlement to the problem and called on other states to follow Russia's example.
Most of Russia's forces pulled out of the rest of Georgia last Friday but it maintains a presence both within the two rebel regions and in buffer zones imposed round their boundaries.