A Florida prosecutor has decided not to press a misdemeanour battery charge against Donald Trump's campaign manager because he may have been attempting to protect the candidate when he roughly grabbed the arm of a female reporter who was asking the Republican candidate questions.

"As state attorney I have made the decision that this office will not be filing charges against Corey Lewandowski for battery," Palm Beach County Florida State Attorney David Aronberg said at a news conference.

Former Breitbart reporter Michelle Fields filed a police complaint against Lewandowski after he roughly grabbed her arm at a press conference in early March and yanked her away from Trump as she was walking alongside the candidate with a notebook and pen in her hand.

While evidence proves that Lewandowski did forcefully grab and yank back Fields – and later lied about it – officials did not believe they could prove a battery case against him beyond reasonable doubt.

There was also a "reasonable hypothesis of innocence" regarding Lewandoski's behaviour, according to Aronberg's deputy, Adrienne Ellis. She said it was conceivable that Lewandowksi grabbed Fields' arm in an attempt to protect the GOP presidential frontrunner from a potential threat. She noted that staff members in campaigns are often given implicit authority from Secret Service agents to help enforce a "protective bubble" around a candidate.

A review of video showed that Fields entered the "protective bubble" around Trump and "brushed or touched" his arm," Ellis said in her memo about the incident.

"When Trump reacted by pulling his arm back and away from her, this reaction could have reasonably led Mr Lewandowski ... to legitimately believe that Ms Fields was making unwanted physical contact with Mr Trump, which caused Mr Lewandowski to react by pulling her away," Ellis concluded.

Aronberg said Lewandowski's lawyers had drafted an apology for Fields but have yet to send it.

"The apology, in a case like this, obviously would be encouraged," Aronberg told CNN. "We always appreciate when people take responsibility for their actions."

Trump had defended Lewandowski's handling of Fields, saying the reporter was grabbing at him, and that Lewandowski moved in to block her. Trump also said the Fields' pen may have held a "little bomb."

Fields may still seek defamation charges against Lewandowski based on his efforts to dismiss her claims and cast doubt on her integrity, a source said.