A Whitehall style guide advises civil servants to avoid using the word "Brexit" and instead refer to "31 December 2020".
The guide says government officials should only use the word when "providing historical context". They are instead advised to refer to the date on which the UK left the European Union.
Officials are also told not to use the term "transition period" but instead use the dates during which Britain negotiated its departure from the bloc.
The British Government's language style guide, published on the Government's official website, states: "You can use the term Brexit to provide historical context, but it's better to use specific dates where possible. For example, use 31 December 2020 rather than Brexit or when the UK left the EU."
Other expressions in the guidelines include "before 31 December 2020" rather than "during the transition period" and "after 1 January 2021" instead of "after the transition period".
Welsh government under fire for Brexit ‘ban’
The news comes after the Labour government in Wales came under fire earlier this week for "banning" the term Brexit and telling civil servants: "Brexit has happened. Use transition period to refer to the time between 1 February and 31 December 2020."
The devolved government's guidelines, which were first published in 2019 and were revised in December, also insist civil servants use "UK government" rather than "HM government".
Speaking to The Times, Andrew RT Davies, the Welsh Conservative Senedd leader, said: "Only last week we had ministers cancelling women from sex education in Wales, and now they're consigning Brexit and Her Majesty to the political correctness bin.
"It's a bonkers misuse of public money and a complete and utter waste of time. Civil servants who are just looking to get on with their day job shouldn't be subjected to such nonsense."
One civil servant said: "It is a massive A-Z, but the only one missing is W for woke. It is just getting so nit-picking –well, if I was allowed to say that."