Some of the new forces could be “coerced” into signing up to the army, they believe, because the Russian president would struggle to get enough volunteers.
The manpower move comes as Ukraine is set to launch a counter-offensive within weeks after Putin’s spring offensive failed to make any significant gains, according to military experts.
However, latest reports suggested Russian forces were gaining more territory in the eastern town of Bakhmut.
In its latest intelligence update, the Ministry of Defence in London said: “Russian media reporting suggests that the authorities are preparing to start a major military recruitment campaign with the aim of signing up an additional 400,000 troops.
“Russia is presenting the campaign as a drive for volunteer, professional personnel, rather than a new, mandatory mobilisation.
“There is a realistic possibility that in practice this distinction will be blurred, and that regional authorities will try to meet their allocated recruitment targets by coercing men to join up.”
The briefing added: “Russian authorities have likely selected a supposedly ‘volunteer model’ to meet their personnel shortfall in order to minimise domestic dissent. It is highly unlikely that the campaign will attract 400,000 genuine volunteers.
“However, rebuilding Russia’s combat power in Ukraine will require more than just personnel; Russia needs more munitions and military equipment supplies than it currently has available.”
The West estimates that Putin’s war, launched in February 2022, has cost up to 200,000 Russian casualties including as many as 60,000 fatalities.
Ukrainian forces have also suffered heavy losses, with thousands of civilians also killed in often indiscriminate Russian artillery shelling, as well as missile and drone attacks.
Putin launched a partial-mobilisation last autumn for 300,000 more soldiers to support his Ukraine invasion.
The US, Britain, Ukraine and its other allies are fighting an information war against Moscow so their briefings need to be treated with caution but are far more believable than propaganda issued by the Kremlin.
Russian forces have had some success in the eastern frontline city of Bakhmut, Ukrainian military officials said, adding that their fighters were still holding on in a months-long battle in which both sides have suffered heavy casualties.
In southern Ukraine, the United Nations nuclear watchdog chief said there had been a significant build-up in the number of troops in the region of the Russian-occupied Zaporizhzhia power plant and it could no longer be protected.
The mining city of Bakhmut and surrounding towns in the eastern industrial region of Donetsk have been the focal point of assault for much of the 13-month-long invasion by Russia of neighbouring Ukraine.
“Enemy forces had a degree of success in their actions aimed at storming the city of Bakhmut,” the General Staff of the Ukrainian armed forces said in a regular report late on Wednesday.
“Our defenders are holding the city and are repelling numerous enemy attacks.”
The average number of daily Russian attacks on the front line reported by Ukraine’s general staff has declined for four straight weeks since the beginning of March, to 69 in the past seven days from 124 in the week of March 1-7. Just 57 attacks were reported on Wednesday.
Russian units were also reported to be digging more trenches ahead of a Ukrainian counter-offensive which is expected to be launched soon.
Ukrainian troops have been training on Western-supplied tanks, including Challenger IIs from Britain, and other heavy weaponry, and building up brigades for the looming advances on Russian lines.
The Zaporizhzhia power station was captured by Russian troops in the opening weeks of the war a year ago and attempts to reduce fighting and shelling around it have failed despite fears of a nuclear disaster.
Rafael Grossi, head of the International Atomic Energy Agency (IAEA), on a repeat visit to the plant on Wednesday, told Russian reporters there had been a “significant increase” in the number of troops in the region.
“It is obvious that military activity is increasing in this whole region. So the plant can’t be protected,” he said.
Mr Grossi said he was putting aside plans for a security zone around the plant so he could propose specific protection measures acceptable to both Russia and Ukraine.
Ukrainian military analyst Oleh Zhdanov, who has served in the military, said that while the offensive on Bakhmut remained intense, “the conclusion is that Russian troops are beginning to rush about from place to place”.
“It now appears that the enemy has shifted its focus to the city itself - that is where the heaviest fighting is now taking place,” Zhdanov said in a YouTube video.
Another Ukrainian military analyst, Roman Svitan, who is also a colonel in the Ukrainian reserves, said the situation in Bakhmut has stabilised and the Ukrainians’ main task there, to destroy Russian forces, was being fulfilled.
Ukraine’s deputy defence minister, Hanna Malyar, said in a social media post that while losses were inevitable “the enemy’s losses are many times greater”.
The Ukrainian military also said there was renewed shelling of Kherson city in the south, along with other towns on the west bank of the Dnipro River that bisects the country.
The Ukrainian air force destroyed a Russian Su-24M bomber, it said. Rocket and artillery in the past 24 hours struck two areas of concentration of Russian forces, an ammunition depot and two fuel depots, it said.