Canada's electoral watchdog has deemed online vote-swapping to be legal but warned that the scheme could be used to dupe voters into casting their ballot for certain parties.

Elections Canada began looking into the issue after a Facebook group surfaced last week urging people living in ridings likely to have tight races to swap votes as a way to keep Stephen Harper's Conservatives from winning a majority.

James Hale, a spokesman for the federal agency, said the act of encouraging someone to vote in a particular way is allowed under the Elections Act.

It's also acceptable for people to invite voters to participate in an organized strategic voting plan, whether on the internet or through other means, he said.

But, he said, "electors should be cautious for a couple of reasons."

Voters could potentially be misled by someone acting under a false identity who tricks them into voting a particular way, which would be an offence under the Elections Act.

Since people cast ballots alone in a voting booth, it is also difficult to verify whether they actually voted as promised.

Also, the agency warned, it is illegal for money or "material benefit" to pass hands as part of a voting arrangement.

"It's not an offence to tell your friend you think they should vote in a certain way, and it's not an offence to say 'Yeah, I think that's a great idea.' It is an offence to accept money or to offer money for that," said Hale.

The Facebook group, titled Anti-Harper Vote Swap Canada, that prompted the Elections Canada investigation was set up by Mat Savelli of Hamilton, Ont.

After its launch a week ago, it quickly ballooned from several hundred members to more than 4,900 as of Wednesday evening.

The group lists 41 ridings likely to be tight races and encourages members to swap votes in order to stop Tories from winning those seats.

Winnipeg South is cited as an example. In that riding, the Liberals have a better chance of beating the Conservatives than the NDP do, so an NDP supporter could agree to vote Liberal in exchange for a Liberal voting NDP in rural Alberta, where the Liberals don't stand a chance.

With files from the Canadian Press