A Federal Court ruling in a case that centres on Prime Minister Justin Trudeau's controversial vacation on the Aga Khan's private island in the Bahamas has opened the door to bringing hundreds of unpaid lobbyists under Canada's lobbying law.

In his 50-page ruling, Justice Patrick Gleeson ordered Commissioner of Lobbying Nancy Bélanger to reconsider her predecessor Karen Shepherd's decision not to investigate a complaint about the trip. Trudeau's January 2017 trip to the island came at a time when the Aga Khan was discussing funding for projects with Trudeau's government.

Conflict of Interest and Ethics Commissioner Mary Dawson found in December 2017 that Trudeau violated ethics rules by accepting the trip to the Aga Khan's island.

However, Shepherd declined to investigate, saying the Aga Khan was not paid to lobby the government and, as a result, did not fall under the Lobbying Act.

Gleeson ruled that Shepherd's definition of "payment" was too narrow.

"The Act's definition of 'payment' might reasonably encompass things of value that fall outside the scope of 'remuneration,'" Gleeson wrote. "For example, and without expressing any view on the question, 'anything of value' might reasonably include a directorship within a corporation or organization, even in circumstances where the position is voluntary."

While the Aga Khan was not paid, he was a member of the Aga Khan Foundation's board. A foundation employee was registered to lobby on behalf of the organization.

Prime Minister Justin Trudeau visited Bell Island in the Bahamas with his family in December 2016 to January 2017. (CBC)

Gleeson said that should have flagged the incident for review.

"I am of the view, in light of the purposes and objectives of the Lobbying Act and the Code and the investigative obligation imposed by section 10.4 of the Act, that the Commissioner was required to take a broad view of the circumstances in addressing the complaint," Gleeson wrote.

"Instead, the record before the Court reflects a narrow, technical and targeted analysis that is lacking in transparency, justification and intelligibility when considered in the context of the Commissioner's duties and functions. The decision is unreasonable."

Gleeson's decision is dated March 29 and was made public by Democracy Watch on Tuesday.

While those who are paid to lobby have to follow stringent rules under the Lobbyists' Code of Conduct, MPs, cabinet ministers and their aides routinely meet with representatives of groups trying to influence them who do not have to register their meetings publicly because they aren't paid to lobby.

In a statement, Democracy Watch co-founder Duff Conacher said Gleeson's ruling "broadens the scope of the Lobbying Act" and closes a loophole in Canada's lobbying rules.

"There are 394 businesses and 581 organizations currently registered in the (lobbying) Registry, and so board members who lobby for any of these businesses or organizations are now required to disclose their lobbying in the Registry."

The terrace overlooking the the Exumas chain of Islands in the Bahamas at Bell Island. (CBC)

Conacher called on the lobbying commissioner to "ensure a full, independent investigation into the Aga Khan's Bahamas trip gifts to Prime Minister Trudeau and Liberal MP Seamus O'Regan in the wake of the ruling."

"Democracy Watch's opinion is, based on the facts and the law and the ruling, it is very likely that the senior officer of the Aga Khan Foundation violated the lobbying code by allowing the Aga Khan to give the trip gifts."

Manon Dion, spokeswoman for the lobbying commissioner's office, said the government is studying the ruling.

"We are awaiting to hear whether the Attorney General will appeal the decision before assessing any next steps."

The justice minister's office referred all questions about a possible appeal to the Privy Council Office, which in turn referred them to the Prime Minister's Office. The PMO has not yet said whether the government will appeal, pointing simply to comments Trudeau made during an event earlier in the day in Kitchener, Ont.

"We trust in the process in place and respect the work that the court and the lobbying commissioner will do," Trudeau told reporters.

Jennifer Pepall, spokeswoman for the Aga Khan Foundation Canada, said the foundation will cooperate with any review by the lobbying commissioner's office.

"Aga Khan Foundation Canada (AKFC) has always rigorously followed the rules of the Lobbying Act and the Lobbyists' Code of Conduct, as well as guidance set out by the Commissioner of Lobbying," she wrote in a statement. "AKFC will continue to cooperate with the Office of the Commissioner of Lobbying in relation to any review undertaken by Commissioner Bélanger."

Philip Cartwright is president of the Government Relations Institute of Canada (GRIC), which represents professional lobbyists. He said the ruling raises significant questions about the rules that govern lobbying by unpaid volunteers and what constitutes a benefit.

"That, to me, is a slippery slope in terms of figuring out how you define ... what's valuable to one person versus another."

If people who lobby on a voluntary basis have to follow the same rules as paid lobbyists, that would make it harder for non-profits and charities to recruit board members, and the number of people obliged to register to lobby would swell, Cartwright said.

"The volume of registrations would absolutely grow, for sure."

Scott Thurlow, an Ottawa lawyer who advises clients on the Lobbying Act, said the law refers to paid lobbyists for a reason.

"I think the court's interpretation of that was too broad, and it is going to inadvertently capture many, many individuals that should not otherwise be captured under the act."

Thurlow said he hopes the government appeals the ruling.

Elizabeth Thompson can be reached at elizabeth.thompson@cbc.ca