Mr Howard said he acted legally

Mr Howard hired his son's firm to send e-mails to voters in his Sydney seat of Bennelong ahead of national elections expected in the coming weeks.

Opposition Labor Party said Mr Howard may have violated his own government's anti-spam laws and urged an inquiry.

Mr Howard said he acted legally as he used his own money for the campaign.

The prime minister - who is seeking a fourth term in office for his Liberal Party - is expected to call the next election for October, analysts say.

'Double standards'

Mr Howard admitted using his son Tim's company, Net Harbour, to send e-mails with the prime minister's letterhead and photograph to voters in Bennelong.

It's not public money, it's private Howard money and I don't intend to disclose that because it's a private matter Australian PM John Howard

The Labor Party said Mr Howard's actions may have breached a law passed earlier this year which banned commercial operators from sending internet users spam.

"Political parties have been given an exemption [by the law] but it seems this Net Harbour is a commercial entity," Labor's Senator Kate Lundy said in a statement.

"John Howard's government banned commercial spamming... but then the prime minister goes ahead and spams the public for political benefit - this is a clear case of double standards," Ms Lundy said.

However, Mr Howard said his party was fully aware of the campaign, adding that he had paid for it out of his own pocket.

"It's not public money, it's private Howard money and I don't intend to disclose that because it's a private matter," he said in an interview with ABC in Tasmania.