The Pride and Prejudice actor took up Italian citizenship on Friday, according to the Rome interior ministry.
Mr Firth said in an interview with an Austrian newspaper: ”Brexit is a disaster of unexpected proportions that does not have a single positive outlook.
“Many colleagues, including Emma Thompson, are, like me, enthusiastic Europeans, and we still cannot believe it.”
The 57-year-old actor has an Italian wife Livia Giuggioli, speaks Italian and spends a lot of his time in the country.
The interior ministry in Rome said: “The celebrated actor, who received an Oscar for his work in the film ‘The King’s Speech’ has married a citizen of our nation and has repeatedly expressed his love for our country.”
The ministry has granted the British actor Italian citizenship saying that he “married an Italian citizen and declared on several occasions his love for the country”.
The Oscar award-winning actor married his Italian wife in 1997 and they have two sons, Luca and Matteo.
The Bridges Jones star Mr Firth has also made films in Italy, including the 2009 movie Genova.
In May, his spokesman said: “Colin applied for dual citizenship, British and Italian, in order to have the same passports as his wife and children
Following the Brexit vote, various EU countries have reported an increase in British nationals seeking a new nationality in the wake of the UK leaving the EU.
British Prime Minister Theresa May has made a keynote speech in Florence to try and revive stalled Brexit talks.