Russia has pushed back the enlistment of 120,000 conscripts by one month as the country struggles to equip, train and deploy its soldiers.
It comes after Vladimir Putin signed an order late last week, which will see Russians aged 18 to 27 not already in the military's reserves being called up for service from 1 November to the end of the year.
On Tuesday the UK's Ministry of Defence (MoD) said the annual autumn conscription cycle was being pushed back a month in a sign of "deficiencies" in Russia's system which will continue to "undermine" its war effort in Ukraine.
This annual cycle is separate from Putin's partial mobilisation order, and the conscripts are not legally permitted to be deployed outside of Russia.
"The late start to the cycle is an indication of growing pressures on Russia's ability to train and equip a large number of new conscripted personnel," the MoD said.
Moscow has called up 300,000 reservists to aid its seven-month campaign in Ukraine, which has seen Russian forces losing swathes of territory and suffering heavy losses in recent weeks.
It comes as Luhansk, Donetsk, Zaporizhzhia and Kherson - four regions of southern and eastern Ukraine - are in the process of being illegally annexed by Moscow.
All four regions are still internationally recognised as part of Ukraine and the referendums Russia used to justify them have been roundly criticised as rigged and illegal.
On Tuesday the upper house of Russia's parliament voted to formally incorporate the regions into its borders, with just a final signature now needed from Putin.
British prime minister Liz Truss said the UK will never accept the regions as "anything other than Ukrainian territory".
In a statement on Friday, she said: "Putin cannot be allowed to alter international borders using brute force. We will ensure he loses this illegal war.
Watch: Chaotic call-up reports add to Russian set-backs
"Putin has, once again, acted in violation of international law with clear disregard for the lives of the Ukrainian people he claims to represent."
Russian forces have been unable to completely hold any of the territories, and Ukrainian forces continue to push through and liberate towns and villages.
On Saturday Ukrainian forces took over the strategic city of Lyman days after Putin declared it as "annexed", causing extra humiliation for the Kremlin.
Since Moscow's draft order, hundreds of thousands of Russian have fled to the border as they do everything they can to avoid being sent to fight in Putin's faltering war.
Last week Putin admitted the "partial mobilisation" has not gone smoothly, following complaints of enlistment officers sending call-up papers to clearly ineligible men.
The draft was billed as enlisting those with military experience and required specialities, but has often appeared oblivious to individuals' service record, health, student status or even age.
Pundits on Russian state TV are now expressing concerns with how the war is going, with deputy chairman of the defence committee Dmitry Sablin admitting the military urgently needs to "stop and regroup".
Appearing on a panel show on Tuesday, the politician said: "We have to understand, no one is going to help us. We don't have and will not have any allies.
"We need to restructure our industry to be self-sufficient to provide our armed forces with all that is needed. The army should not be experiencing lack of munitions, missiles, mortars."
Sablin adds: "We also need to restructure journalism. We need to impose censorship and not be afraid of it. Of course, there should be freedom and all that, but we have to understand that we live in hard times.
"We need to restructure our military command system... We're seeing all the shortcomings right now, we should openly talk about all that is happening and openly draw conclusions in order to win."
Painting the war as a fight between Moscow and the West, rather than an invasion of a sovereign country, he tells viewers: "The war is happening over Russia, with our very existence on the line - to be or not to be.
"That's why I'm convinced we will win either way, we have no other options."