A federal judge Thursday denied a Willis Towers Watson P.L.C. unit’s petition for a temporary restraining order against Brent Hartman, a construction insurance broker, and rival Alliant Insurance Services Inc., which Mr. Hartman joined last week.

The ruling is part of a growing legal fight between the two brokerage firms that was sparked when Alliant swooped in to recruit several construction brokers earlier this month.

A judge in Georgia is due to hold a separate hearing in a related case next week.

In a filing in U.S. District Court in Florida on Wednesday, Willis Towers Watson sought unspecified compensatory damages and a temporary restraining order barring Mr. Hartman and Alliant from soliciting Willis Towers Watson clients and from Mr. Hartman working for Alliant until he had served out a 15-day notice period.

The complaint, filed by Willis Towers Watson unit Willis Insurance Services of Georgia Inc., alleges that Newport Beach, California-based Alliant is attempting “to decimate at a nationwide level Willis’s construction industry practice group.”

In addition to Mr. Hartman, who was named a managing director at Alliant, Alliant recruited Bill Noonan, who was also named a managing director. Willis Towers Watson sued Atlanta-based Mr. Noonan earlier this week charging he had breached confidentially agreements.

In its suit against Mr. Hartman and Alliant, Willis charges that “Alliant is attempting to steal millions of dollars of revenues from Willis by convincing Willis employees to not honor their contractual obligations to Willis in their employment agreement. Alliant and Hartman are also seeking to declare that Hartman’s restrictive covenants are unenforceable in an action filed in Georgia.”

An emergency hearing in the Georgia case is slated for Aug. 2. Willis sued Mr. Hartman and Alliant in Florida because it said Mr. Hartman has been based there since 2016.

Willis alleges that Alliant’s “raid” had been planned for months and that since Mr. Hartman’s and Mr. Noonan’s resignations, nine other staff in Florida, Georgia and Minnesota have resigned and that Alliant targeted 15 other Willis staff. Willis Towers Watson units filed lawsuits this week in four separate jurisdictions to “protect their interests,” the suit says.

In ruling against Willis in Willis Insurance Services of Georgia Inc. v. Alliant Insurance Services Inc. and Brent Hartman, Judge Steven D. Merryday for the U.S. District Court in Tampa, Florida, said that Mr. Hartman’s employment agreement may not require him to provide 15 days’ notice.

The agreement states that it shall “continue until terminated … by either party, with or without cause, upon fifteen calendar days prior written notice,” court papers say.

“Willis equates termination and resignation, which is an equivalency that is far from certain,” Judge Merryday wrote.

The judge also found that the nonsolicitation agreement that Mr. Hartman had with Willis may prove to be too broad to be enforceable in various states.

“For example, rather than prohibiting only Hartman’s soliciting a prospective or current Willis customer, the agreement prohibits Hartman’s accepting business from a prospective or current Willis customer — even if Hartman never attempted to persuade the customer to switch to Alliant,” the judge wrote.

In addition, the agreement as written would expand to cover geographies where Willis expands its business, the judge ruled.

Finally, the judge ruled that “Willis fails to identify persuasively the governing law.” Mr. Hartman signed the employment agreement while assigned to an office in Georgia and later transferred to Florida, the judge notes, and has already started his own legal proceedings against Willis in Georgia.

“Because the applicability of Florida law appears uncertain and because Willis offers no argument about the enforceability of the provisions under Georgia law, Willis fails to show a demonstrable likelihood of success on the merits,” the judge ruled.