In an interview by the Guardian's Rajeev Syal - and also covered in the Guardian Brexit weekly briefing - General Secretary Dave Penman has said that the Government is "trying to deny the complexity" of Brexit.
He added: "When anyone pops their head above the parapet - former Permanent Secretaries, ex-Cabinet Secretaries, the Institute for Government - and says this is going to take a long time and it's complex, they are immediately shot down and accused of betraying the will of the people. The politics around Brexit are the biggest risk to Brexit."
Penman said that the civil service "is either going to have to be given more resources to deal with Brexit and its usual work or it will have to change its priorities. And the Government doesn't want to admit to either".
He also warned about the dangers of not facing up to these consequences: "Either Brexit is not going to be funded and resourced or the PM is going to have to drop something. Something is going to have to give, and it is not going to be Brexit.
"The civil service will have to effectively run a Formula One car whilst building next year's car at the same time. It can be done, but it's going to be a bumpy ride."
Read the full article:
Theresa May 'lacks courage to admit complexity of Brexit'
Penman was also interviewed by BBC Radio 4 on the same issue, as part of its Six O'Clock News programme on 28 December, with the story also being reported on the BBC News website.
Penman said that it "requires courage from politicians to admit Brexit will be difficult and take resource, and we've not seen this so far".
Hear the full piece:
BBC Radio 4 Six O'Clock News: 28 December 2016
(9 minutes 5 seconds into the programme)