Russia would be annexing at least 90,000 square km - an area around the size of Hungary or Portugal.
Russia could declare the annexation of 15 per cent of Ukraine on Friday as Moscow-backed "referendums" end, according to British officials.
The polls -- rejected as shams by the West -- are closing today in four Russian-controlled regions of Ukraine.
President Vladimir Putin will address both houses of the Russian parliament on Friday when he could officially declare the annexation, the British Defence Ministry said.
Taken together, Russia would be annexing at least 90,000 square km of Ukrainian territory - an area around the size of Hungary or Portugal.
Moscow announced annexation polls last week for people living in Donetsk, Luhansk, Kherson and Zaporizhzhia, located in south and east Ukraine.
Western powers, as well as Kyiv, have said they will not recognise the outcome of the "referendums" amid claims of residents being coerced into voting.
How fast annexation could happen - a timeline
After the "referendums" -- which are expected to support joining Russia -- close on Tuesday, the leaders of each region could make official annexation requests to the Kremlin.
A law to formally recognise the four regions as Russian territory could be submitted to Russia's parliament on Tuesday evening, according to local media, which report the bill could be adopted by Wednesday.
It would then pass by the Federation Council -- often seen as a formality and one that could be completed by Wednesday or Thursday.
Putin is expected to make an official announcement on the annexation by the end of the week.
TASS, a Russian state-owned news agency, reported that senators had been told to prepare for an "important event" on Friday.
It said that politicians were required to take three COVID tests beforehand, a health measure that people meeting Putin must take, according to AFP.
However, Putin's announcement could come sooner.
In 2014, he signed a bill recognising Crimea as part of Russia two days after the Moscow-organised "referendum" and before a vote by parliament.
What does annexation mean for the war?
Ukraine's president Volodymyr Zelenskyy has repeatedly warned that "pseudo-referendums" on annexation by Russia would destroy any chance of peace talks.
Some fear that, by formally annexing Ukrainian territories, Putin is giving himself the potential pretext to use nuclear weapons.
Russia's nuclear doctrine allows the use of such weapons if weapons of mass destruction are used against its territory or if the Russian state faces an existential threat from conventional weapons.
British officials say the annexation may consolidate support for the war among the Russian public.
“Russia’s leaders almost certainly hope that any accession announcement will be seen as a vindication of the ‘special military operation’ and will consolidate patriotic support for the conflict,” a spokesperson for the British Defence Ministry said on Tuesday.
“This aspiration will likely be undermined by the increasing domestic awareness of Russia’s recent battlefield [setbacks] and significant unease about the partial mobilization announced last week.”