A spokesperson said the UK Home Office is "urgently looking into why this happened"
An investigation has been launched in the UK after 100 deportation letters were sent in error to EU citizens living in the UK.
A spokesperson for the UK Home Office said "a limited number of letters" were sent by mistake and the department is "urgently looking into why this happened."
They added: "We are contacting everyone who received this letter to clarify that they can disregard it.
"We are absolutely clear that the rights of EU nationals living in the UK remain unchanged."
One of those who received a letter was Finnish academic Eva Johanna Holmberg, who was told she had a month to leave and warned she was "liable to be detained."
She wrote in a Facebook post that the "absurd nonsense has aged me at least five years" and added the ordeal has made her "even less likely to trust anything Amber Rudd, Theresa May, or David Davis says to calm us EU nationals down."
Pro-EU campaign group Open Britain described the letter as "shameful stuff from the same department that gave us the disgraceful 'go home vans' a few years ago."
Executive director James McGrory said: "It's little wonder that many EU citizens feel worried about their future status in the UK when they hear of people with every right to be here getting letters threatening their deportation.
"The Home Office show scant sign of being able to get a completely new immigration regime up and running by 2019.
"Ministers should drop their damaging rhetoric about an immigration target and instead come forward with proposals that recognise the work of EU nationals already in this country, and others who we will undoubtedly need in the future."
The Liberal Democrats have called on Home Secretary Amber Rudd to apologise and said the episode "shames Britain."
The party's home affairs spokesman Sir Ed Davey said: "EU nationals who have made their lives here are already facing huge uncertainty over Brexit.
"It is appalling that some are now being officially threatened with deportation.
"This catastrophic error is a sign of the chaos and incompetence at the heart of this Conservative government."