Four occupied areas of Ukraine will become part of Russia on Friday, the Kremlin has said, and any attack on them will be considered an attack on Russia itself.
It comes after so-called referendums, which the West dismissed as an illegal sham.
President Putin will attend a ceremony in the Kremlin tomorrow at which documents will be signed and the regions 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 amount to about 15% of Ukrainian territory.
Giant video screens were set up in Moscow’s Red Square on Wednesday, with billboards proclaiming “Donetsk, Luhansk, Zaporizhzhia, Kherson – Russia!”
The voting in the south and east of the country began last Friday, often with armed officials going door to door collecting ballots.
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.
A concert in Red Square is also planned on Friday evening.
‘Radical change’ after referendums
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 in 2014.
James Cleverly, the foreign secretary, 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.”
Mr Cleverly said that in parts of occupied Ukraine civilians are tortured, arbitrarily detained and forcibly deported to Russia.
The voting is believed to have been mainly prompted by Ukrainian counterattacks that have won back swathes of occupied territory.
While much of Luhansk is under Russian control, about 40% of Donetsk is not.
Large parts of Kherson and Zaporizhzhia are also under Ukrainian control. Fighting continues along the frontlines in all the regions.