By Morag MacKinnon

PERTH, Australia (Reuters) - Australian Prime Minister Julia Gillard's government was rocked by the surprise resignation of two of her most senior cabinet ministers on Saturday, just four days after she stunned voters by calling a national election in eight months' time.

Close ally Nicola Roxon quit as Attorney-General along with Chris Evans, who as Senate leader is the third in line to the prime ministership, citing demands of the job and family reasons for their decisions.

Australian media described the resignations as a severe blow for the ruling Labor Party, saying they undermined Gillard's message of stability and a lack of confidence in ability to win a third term on September 14.

Gillard put on a brave face at a news conference in Canberra, saying she had known of the impending resignations for some time.

"We will be able to present the Australian people with a rejuvenated team as we move into the parliamentary year of 2013," a teary Gillard said.

Earlier this week Gillard broke with convention and surprised voters by announcing a far-off election date, an unusual move that she said was designed to end political instability.

While the resignations will not impact the minority hold that Gillard's Labor government has on power, they will add to the uncertainty surrounding her government which relies on a handful of independents and Greens to command a one-seat majority.

The Sydney Morning Herald wrote on its website that the surprise departure of two of the government's most senior figures was "a very bad look".

Opinion polls show Tony Abbott's opposition Liberal-National party well ahead of the government and Gillard would be swept from office, losing up to 18 seats, if an election were held now.

The election will decide whether Australia keeps its controversial carbon tax, and a 30 percent tax on coal and iron ore mining profits, which Abbott has promised to scrap it if he wins power.

Roxon, Australia's first female Attorney-General, notably took on the big tobacco companies with plain packaging legislation last year and won.

The law that states cigarette and tobacco products must be sold in plain olive green packets with graphic health warnings is being closely watched around the world and could have a major effect if adopted as a precedent in other countries.

(Editing by Jeremy Laurence)


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