Reform UK has pledged to save £100mn a year by closing expensive government buildings while cutting a civil service "wasteland", in a major overhaul of Whitehall that would make hundreds of thousands redundant.

Danny Kruger, who earlier this year left the Conservatives to join the party that is leading in the polls, said a Reform government would cut the size of the civil service, which has ballooned since Brexit to more than 540,000 staff, by a third.

"Britain used to administer an empire from a company building around Downing Street in the 1820s," the Reform MP for East Wiltshire said during a press conference on Tuesday. "Now, in 2025, it is a wasteland."

As part of a wider plan to shake-up of Whitehall, the party would bring in outsiders to fill senior positions and privatise core services, such as prison management and employment support. It would also make it easier for politicians to sack failing officials.

"We expect the headcount to fall dramatically. Ten years ago civil service was 30 per cent smaller than it is today. So we can at least get back to that size but I'm confident we can go further," Kruger said.

"The productivity crisis in the UK is absolutely shameful in the age of the internet," he said. It would also bid to automate several state functions, he added without specifying.

The populist party also pledged to reform what it described as "clearly unfunctional" Cabinet Office, which is responsible for ensuring the effective running of government, in an effort to cut costs, but came short of specifying how it would do so.

Kruger said by closing civil service office buildings in London when leases come to an end, and moving officials back to their traditional home of Whitehall, the party would be able to save as much as £100mn a year.

However, Labour announced in May that major Whitehall government buildings are to be shut as they seek to shed 12,000 civil servant jobs in London.

On Tuesday, chair of the Labour party Anna Turley MP said: "You can't save money by closing government buildings that are already closing . . . It's deeply embarrassing for Nigel Farage that his new policy chief's first policy has already fallen apart."

During the press conference, Reform's chief of policy, Zia Yusuf, said Kent county council had identified "northwards" of £60mn in savings during its first six months.

Antony Hook, leader of the Liberal Democrat group at Kent council, said the party seemed to lack "basic financial literacy", adding that it appeared to be "confused" by the difference between recurring and one-off savings.

The council has suffered a troubled week in which Reform has expelled four of its Kent councillors after a video was leaked that revealed bitter infighting among the council's leadership group.

Reform pointed to a raft of proposed efficiencies at the authority, including a £5.5mn discount on a Barclays loan it has repaid early.

"By my reckoning that is the largest amount of savings ever found by a council in the UK outside . . . the bankruptcy process," said Yusuf.

Birmingham council cut £212mn from its annual budget in 2011, while Manchester council trimmed its budget by £109mn the same year.

When asked to clarify Yusuf's claim, Reform said £63mn in savings found in Kent during Reform's first six months would equate to £126mn when "annualised".

Reform UK vows to cut civil service 'wasteland' to raise efficiency