Britney Spears Alleges Former Bodyguard Hacked Into Her iCloud Account

Britney Spears has major beef with an ex-bodyguard who allegedly hacked into her personal devices and Apple iCloud account ... and now she's threatening legal action.

The singer’s lawyer fired off a cease-and-desist letter, obtained by TMZ, to Thomas Bunbury ... who she claims gained access to her devices and iCloud account without her authorization or consent.

In the letter, dated last month, Brit says Bunbury's allegedly violating a bunch of state and federal laws ... and she's threatening to report him to law enforcement and press charges.

She says he would be wise to stay the hell away from her personal devices and accounts, delete any copies he might have made -- and, if he sent any of her photos, files or docs somewhere else ... she needs to know.

Sources familiar with the situation told TMZ that … Bunbury was fired in August for allegedly breaking his NDA by communicating with fans and media outlets.

We're also told the alleged hacking went down after he was fired, and Britney was locked out of her accounts at various times.