TORONTO -- Some of the candidates competing to head Ontario's Progressive Conservatives are raising questions about the validity of some new memberships as the party extends the deadline for members to register to vote for a new leader.

At least two candidates -- former Toronto city councillor Doug Ford and Toronto lawyer Caroline Mulroney -- are sounding the alarm about the use of prepaid credit cards in purchasing new memberships during the leadership campaign.

Ford's camp says payments made through prepaid cards cannot be tracked, which makes it impossible to verify that the person buying the membership is the person who casts a ballot. His campaign is asking for all memberships paid for with prepaid credit cards to be removed from the list of eligible voters for the leadership vote.

Mulroney, meanwhile, says on Twitter that her campaign has flagged issues related to the use of so-called "burner" cards to buy multiple memberships, and to registrations done by mail.

She says the party has already removed 500 "highly suspicious" members from the list of eligible voters for the leadership race, and she believes the party should continue to investigate.

The party said it could not comment on the specific allegations at this time but said it is monitoring for suspicious activity.

Meanwhile, Tory members have been granted an extra three days to sign up to vote. The deadline was originally set for Friday and has now been moved to 11:59 p.m. on March 5.

Hartley Lefton, chair of the party's leadership organizing committee, said the date was pushed back after the party realized it could process registrations faster than it initially believed.

Voting is set to take place online between March 2 and March 8, with the results announced on March 10.

Four candidates are currently vying for the top job -- Ford, Mulroney, former Tory legislator Christine Elliott, and parental rights activist Tanya Granic Allen. They will face off tonight in a final leadership debate to be held in Ottawa.

The contest was triggered by the resignation of Patrick Brown, who stepped down last month amid sexual misconduct allegations.

Brown vehemently denies the allegations and even briefly re-entered the race to reclaim his old job, but pulled out earlier this week, saying the race was taking a toll on his friends and family.