FDA General Secretary Dave Penman appeared on Thursday's BBC Breakfast programme on BBC One, discussing the impact of yesterday's Spending Review announcement by the Chancellor.
Penman told business journalist Steph McGovern: "The estimate is that there will be around 100,000 job losses in the public sector. For us the concern is about maintaining public services with such dramatic cuts in public spending.
"In the Home Office the admin budget's going to be cut by 30%, in the Ministry of Justice that's going to be 50%… The Chancellor needs to be honest with taxpayers and public servants about what can no longer be done.
BBC Breakfast is not available online for rights reasons.
The FDA issued a press release responding to yesterday's Spending Review announcement, which was quoted in The Guardian and Civil Service World.
Penman said: "Whilst these budget cuts have been extensively trailed over the last few months, the sheer scale of reductions in public spending announced today is no less eye watering.
"Despite months - if not years - of preparation, the Government has once again failed to demonstrate, to both public servants and to taxpayers, how it will cut resources so dramatically while ensuring public services are maintained.
"The Chancellor's announcement today means that many departments are facing cuts of around 50% over a decade. Efficiencies of this scale cannot be achieved through the usual salami-slicing approach favoured by the Treasury and Chancellor.
"Unless public services are transformed, these budget cuts can only result in the decimation of key public services.
"The question has to be asked: how can the billions of pounds of capital investment announced by the Chancellor be delivered efficiently by a civil service stripped of resources and people?"
The Association of Revenue and Customs (ARC) - FDA's section representing professionals in HMRC - also issued a press release in response to the Spending Review.
President Tony Wallace said that while ARC was "pleased to see that its case for a reinvestment in skills has at least been partially recognised by the Chancellor" after a £2.1bn HMRC reinvestment package was announced, he also voiced concern over HMRC's budget being "cut by a further 15% by 2020".