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I am not afraid of any man, not even the prime minister
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Judge Mishael Cheshin
The head of Central Election Committee (CEC), Judge Mishael Cheshin, says he was forced to act because Mr Sharon was engaged in "political propaganda".
The prime minister had called a news conference to rebut allegations of financial malpractice and corruption, but was taken off the air after about 10 minutes.
Mr Sharon - who is standing for re-election on 28 January - has seen his popularity fall sharply after the Israeli daily Ha'aretz revealed last week that the attorney general was investigating a $1.5m loan the Sharon family received from a South African friend.
'Not afraid'
Judge Cheshin was quoted by the Ha'aretz on Sunday as saying that at the beginning of Mr Sharon's press conference he realised that it was blatant propaganda.
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I have nothing to hide
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Ariel Sharon
"As far as I understand Sharon planned it in advance and even read from notes he made."
"Had I not stopped him I would have betrayed my duty to uphold the law. I am not afraid of any man, not even the prime minister."
"If I had to do it again I would make the same decision," Judge Cheshin told AFP.
The meeting of the CEC was held after demands from both Mr Sharon's Likud Party and the opposition Labour Party.
The Likud Party said Mr Sharon was prevented from defending himself in the face of a very damaging scandal.
For its part, Labour has demanded equal time to rebut the charges of corruption which Mr Sharon launched against their leader Amram Mitzna.
Israeli broadcasting law forbids "election propaganda" from being aired except in special slots for a month before elections.
Attack on Labour
On Thursday, Mr Sharon began his defence on television by accusing the Labour Party of a smear campaign designed to topple the government.
Ten minutes into his speech, Judge Cheshin ordered that Mr Sharon be taken off air.
Mr Sharon had just started to address the specific allegations against him, having called Labour's behaviour at such a critical time "criminal" and "irresponsible".