Detailed plans to cut holidays, lengthen working weeks and reduce flexible working for 450,000 public employees have been disclosed for the first time in leaked Cabinet Office documents.

A letter sent to every human resources director across the civil service, and seen by the Guardian, outlines the scope and speed of the coalition government's planned reforms and how they would affect staff.

By the end of this year, directors in every Whitehall department are expected to have examined the terms and conditions of their workforce and outlined plans to make their jobs more like those in the private sector.

The documents reveal several aspects of working life as susceptible to radical change, including employees' annual leave, occasional days' leave, sick pay, hours of work, the ability of employees to move from one job to another and probationary periods.

Managers have also been ordered to review policies including the family-friendly scheme of flexitime, travel and expenses, disciplinary procedures and performance management. The letter applies to staff below the level of senior civil servants.

Mark Serwotka, general secretary of the Public and Commercial Services Union, said the government's plans to rush through fundamental changes would be "a sickening blow" for many public sector workers. "Amid an imposed pay freeze, and cuts to pensions and redundancy terms, the Cabinet Office now wants to undermine some very basic working conditions that any decent employer should offer," he said.

"It is impossible to separate this from the Tory-led government's wider political project to unpick the welfare state and drive down pay, conditions and employment rights across the economy, and we are determined to oppose it at every step."

The leaked letter was sent two weeks ago by the director of the civil service human resources and capability group, William Hague (not to be confused with the foreign secretary).

"The civil service reform plan states that each department will undertake a review of their terms and conditions. Your review should ensure that your department, and collectively the civil service, continues to be a good employer, offering terms and conditions comparable with, but not beyond what a good modern employer would provide," he wrote.

With a nod to rightwing attempts to paint public sector jobs as overpaid and comfortable, Hague wrote: "This is our opportunity to tackle those terms and conditions where we have been less responsive in the past as well as those that have left the civil service open to caricature."

He suggested that staff would have to become more "flexible and collaborative" in a transformed civil service, which may result in some having to move to keep their jobs.

"One feature of your review of terms and conditions should be to promote greater mobility. This will open up opportunities for employees to develop and build expertise," he wrote. "We wish to offer terms that reflect best practice in the private sector rather than the average."

The documents outline "other areas for consideration" which could also face cuts.

These include a number of family-friendly policies such as childcare, compressed hours working, which is a form of flexitime, working from home, parental leave, part-time working, job sharing and term-time working, which allows staff to organise their working hours around school holidays and voluntary work.

A list of other policies that could be reviewed, at the discretion of human resources directors, include gifts and hospitality, whistleblowing, apprenticeships, work experience, advances of pay, allowances, excess hours, weekend working, travelling time, eye tests, legal representation paid from the public purse, relocation fees, reward vouchers for good work, and advances in money to pay for travel expenditure and subsistence payments.

Directors are asked to consider other possible ways of saving money, beyond the lists drawn up by the Cabinet Office: "These lists are not definitive, and departments should include any terms and conditions, policies and practices or pay-related terms deemed relevant."

The review process has been split into three phases to be completed by the end of this year: an assessment of terms and conditions to be submitted by 19 October; a draft implementation of changes and a plan with key dates to be submitted by 16 November; and an implementation strategy and plan to be submitted by 31 December.

"Details of who the changes are to be applied to should be included in this document along with key dates, savings/costs the department has identified and the evidence it has relied on in reaching its conclusions," the documents state.

The aim is to deliver all reforms over two years, starting in April 2013.

A Cabinet Office spokesperson said the civil service was being reformed to make it faster, more unified, focused on outcomes not process, and ultimately more enjoyable to work for: "Because we want to attract the best staff, we will remain an employer with good terms and conditions, as we have always been. However, while there has been significant recent change in pay and pensions, there are other terms and conditions that have not been updated and are now outliers compared to best practice. We will address this and ensure a modern employment offer is available to all."