Whitehall consultants on £1,000-£2,000 per day
At least 30 consultants worked in government departments last year on day rates of £1,000-£2,000, figures obtained by the BBC's Newsnight show.
The Department for Transport took on 10 consultants, while the Ministry of Defence said it had recently employed someone earning up to £3,000 a day.
The government said it paid £317m to consultants last year, and spending on them had dropped by 75% since 2009.
But unions and campaigners questioned the fees and called for more details.
Jonathan Isaby, chief executive of the Taxpayers' Alliance, said: "I think people will be extremely concerned about the number of consultants being paid vast amounts of money for a day's work in government.
"Government is rightly reducing the headcount of the civil service but you cannot negate that by paying huge amounts to consultants instead."
He added: "I think there are big questions about whether we are getting value for money, not least because there is a severe lack of transparency about what they are actually doing."
The figures, obtained under the Freedom of Information Act, show that five government departments hired the 30 consultants.
'Right to know'
Apart from those at the Department of Transport, eight worked at the Ministry of Justice, five at the Department of Energy and Climate Change, four at the Home Office, two at the Treasury, and one at the Ministry of Defence.
Separate figures show the MoD spent £137m on "technical consultants" last year - money that was not included in the government's £317m total.
Leslie Manasseh, deputy general secretary of Prospect, the union which represents professionals across the civil service, said there was confusion about how the government calculated its figures.
He said government cuts in Whitehall had forced departments to hire a "growing" army of consultants and other managers - and little was known about them.
"This is taxpayers' money and they have a right to know how it is being spent," he said.
"We know how much the prime minister earns, we know how much senior civil servants and other politicians earn, we have little or no idea how much individual consultants are earning."
The government would not give details about the number of consultants earning more than £1,000 a day, but it said it had brought this kind of spending under control.
A spokesman said: "This government has scrutinised spend by departments like never before.
"Already it has put an end to excessive spending on consultants and interim staff by establishing stringent controls.
"Certain departments do, however, have a requirement for specialist roles, especially where they are undertaking complex transformative projects.
"Such roles are only authorised where the skills are not readily available within the civil service."