Four occupied areas of Ukraine will become part of Russia on Friday, the Kremlin has said.
It comes after the holding of so-called referendums, dismissed by the West as an illegal sham.
President Putin will attend a ceremony in the Kremlin tomorrow at which documents will be signed and they will be incorporated into Russia.
Pro-Kremlin officials said on Wednesday that the self-declared republics of Luhansk and Donetsk and the Kherson and Zaporizhzhia regions had all voted to join Russia.
It claimed 93% of votes in Zaporizhzhia were in favour, as were 87% in Kherson, 98% in Luhansk and 99% in Donetsk.
The regions together amount to about 15% of Ukrainian territory.
The referendums in the south and east of the country began on 23 September, often with armed officials going door to door collecting votes.
It was virtually certain such results would be returned.
President Putin will give a speech after Friday’s ceremony and meet Moscow-appointed administrators of the regions, the Kremlin said.
Russia is now expected to claim that efforts by Ukrainian forces to recapture these areas are an attack on the motherland.
President Putin has warned he would be prepared to use nuclear weapons if Russia’s territory is threatened.
Mr Peskov said the legal situation would “radically change” after the voting, “with all the corresponding consequences for protection of those areas and ensuring their security”.
Russia’s organising of ‘referendums’ is the same tactic it used to claim Ukraine’s Crimea region as its own in 2014.
UK Foreign Secretary James Cleverly told the United Nations last week: “We know what Vladimir Putin is doing.
“He is planning to fabricate the outcome of those referenda, he is planning to use that to annex sovereign Ukrainian territory, and he is planning to use it as a further pretext to escalate his aggression.”