The head of PBS has confirmed she will not seek reappointment and will step down when her term ends.

Her move has triggered calls for a change in the network's funding model, to make it safe from political interference.

Pat Mitchell was under fire for spending public money on a cartoon show, Postcards from Buster, that featured two real-life lesbian couples in an episode.

Mitchell, 62, who joined PBS in 2000 and has led it to notable successes, told PBS managers at the network's annual meeting Monday that she would not seek a third three-year term after her current contract ends in June 2006.

In her media statement, Mitchell made no reference to the Buster controversy. PBS said it was "blatantly incorrect" to connect her departure with the controversy.

The American public broadcaster pulled the episode of Postcards from Buster after U.S. Education Secretary Margaret Spellings contended that the episode did not fulfill the intent Congress had for programming.

Spellings said many parents would not want children exposed to such lifestyles. Mitchell was then also criticized by liberals for caving in to conservative attacks.

WGBH-TV, the Boston public television station that produced the series in conjunction with a Canadian company, broadcast the episode anyway and made it available to PBS affiliates. Forty-seven American stations nationwide showed it.

In Postcards From Buster, the title character, an animated bunny named Buster, travelled to Vermont, a state known for recognizing same-sex civil unions. Though the focus was on farm life and maple sugaring, the episode, entitled "Sugartime!", featured two lesbian couples.

Brigid Sullivan, WGBH's vice president of children's programming, told the Boston Globe that the future of Postcards was now in jeopardy as the station awaited word from PBS about funding for a second season.

"Normally, we would have heard something in January," she said. "We're existing on fumes now."

New York television producer Judy Crichton warned about the wider impact of the Buster fallout. She said in the current American political climate, "there'll be this kind of self-censorship that begins to take place as producers begin to play it careful and second-guess themselves."

A WGBH board member said Congress should consider creating a new business model for public television that would protect the network from political pressure by changing its source of funding.

The creation of a national trust fund or endowment would allow PBS to be free of the whims of the White House, said Peggy Charren, a children's television advocate and WGBH board member.

"This is not a matter of looking for the right leader for PBS," she said. "The people in power want to control the network if they can. The important thing is to fix it so they can't. Annual giving from the government is a prescription for disaster."

PBS depends on Congress for funding, and also relies on contributions from individuals, local stations, state governments and foundations. The network receives money for the Postcards from Buster series through the U.S. federal Ready-To-Learn program aimed at helping young children learn through television.

The political controversy over the program meant the always cash-strapped not-for-profit organization was also potentially facing significant financial challenges.

Under Mitchell, PBS's prime-time ratings rose to the highest ever and PBS stations extended the reach of its digital channels to more than 89 per cent of the U.S.

She has added diversity to the schedule – including American Family an emmy-nominated series featuring a Latino family, and American Mystery!, a special featuring Indians living in the Southwest.

Postcards from Buster, which also focuses on diversity, had its premiere in the fall of 2004. A spinoff of the hit animation series Arthur, it is co-produced by Canadian animation company Cookie Jar Entertainment, formerly known as Cinar.