Carter, 87, was speaking after initial vote tallies put the Brotherhood's candidate ahead in the first round of Egypt's presidential election, which his Carter Center helped monitor.
- Leading Egypt candidate calls 'racist' Israel a threat
Tantawi: Egypt ready to do battle
- Carter warns against war with Iran
The US statesman, who brought together Israeli leader Menachem Begin and Egypt's Anwar Sadat in 1978 to agree the Camp David accords which led to a 1979 treaty, said he had held long discussions with senior Brotherhood figures in Egypt this week.
"My opinion is that the treaty will not be modified in any unilateral way," Carter said at a news conference in Cairo to present the preliminary findings of his election monitors.
Carter with Begin (L) and Sadat (Photo: Fisher Klingenstein)
Official results in Egypt's first free leadership election are due on Tuesday, but informal tallies put the Brotherhood's Mohamed Mursi and Mubarak's last Prime Minister Ahmed Shafiq in the lead. If confirmed, they would fight a run-off in June.
Hamdeen Sabahy, a leftist who has championed Palestinian resistance against Israel, was running a close third.
The peace treaty remains a lynchpin of the US' Middle East policy and, despite its unpopularity with many Egyptians, was staunchly upheld by President Hosni Mubarak until his overthrow last year in a popular uprising.
The Brotherhood, long suppressed under Mubarak, is vehemently critical of Israel, and its Palestinian offshoot Hamas rules the Gaza Strip. Israeli officials have watched political turmoil since Mubarak's overthrow with growing wariness.
Mubarak's fall opened up a freer form of Egyptian politics in which the popular mood looms far larger.
Mursi criticizes Israel but says he would respect the treaty. One of his aides said Mursi would not meet Israeli officials as president, though he might delegate that task.
Cairo needs good ties with Israel's closest ally the United States, which provides billions of dollars in military and civilian aid and is pressing other major foreign donors to support Egypt's struggling economy.
But some of the election contenders said the peace treaty should be reviewed, partly because of perceptions the deal Carter brokered was biased in Israel's favour.
Carter said the treaty had not been violated by either side since its inception and that any problems had been resolved peacefully, including a flare-up of tension last year over the killing of some Egyptian border guards.
"The Israelis apologised for that. They see great value in preserving the treaty," said Carter.
The Camp David accords were also supposed to guarantee the rights of the Palestinians, at Sadat's insistence, but that aspect had not been honored, Carter said.
- Receive Ynetnews updates
directly to your desktop