The former head of the civil service has warned Theresa May that Britain is in for a "tough ride" unless cabinet ministers end their squabbling over Brexit.
Lord O'Donnell called for ministers to unite in backing a lengthy transition deal and "start being honest about the complexity of the challenge" after the UK leaves in 2019.
Writing in The Observer, he warned: “The EU has clear negotiating guidelines, while it appears that cabinet members haven’t yet finished negotiating with each other, never mind the EU."
He wrote: "There is no way all these changes will happen smoothly and absolutely no chance that all the details will be hammered out in 20 months.
"We will need a long transition phase, and the time needed does not diminish by pretending that this phase is just about “implementing” agreed policies as they will not all be agreed."
The former Cabinet Secretary added: "So fasten your seat belts, we are in for a rough ride."
It comes after Tony Blair intervened to say Brexit could be stopped if Britain's leaders realise EU officials are prepraed to "meet us halfway" on restricting the free movement of people.
The former Labour Prime Minister said it is possible the will of the British people could change as the public becomes more aware of the potential economic damage of a hard Brexit.
The Chancellor, Philip Hammond, has also called for colleagues to accept the need for a lengthy transitional period.
Defending the Government's plans, a spokeswoman for the Department for Exiting the European Union told The Guardian: “Our aim is to build a deep and special partnership with the European Union, as the EU’s closest friend and neighbour.
"As we negotiate to leave the EU, we are seeking the best possible deal for the UK, one that gives citizens and businesses as much certainty as possible.”
