He is the most senior surviving member of the Khmer Rouge, the group blamed for the deaths of up to two million Cambodians during its rule in the 1970s.
Son Arun, Nuon Chea's lawyer, said after the ruing that he regretted the decision as his client's mental and physical health had deteriorated since his arrest.
"His health is weakening and he is forgetting a lot," he Arun said, adding that he had asked the court to determine whether Nuon Chea would be mentally fit to stand trial.
Launching his appeal against his continued detention last month, Nuon Chea denied prosecution suggestions that he might try to flee the country or interfere with possible witnesses.
Born in 1923, Nuon Chea is the most senior surviving former Khmer Rouge leader
Served as deputy to Khmer Rouge leader, Pol Pot, and was group's chief ideologue for more than 30 years
Played a key role in carrying out Khmer Rouge plan to relocate millions of Cambodians to vast collective farms, which later became the notorious "killing fields"
Arrested in September 2007, at his home in Pailin near Thai border
Facing charges of war crimes and crimes against humanity
"I have no desire to leave my beloved country," he told the court.
He says his arrest last September was an "illegal act".
The former right-hand man to Pol Pot, the Khmer Rouge leader who died in 1998, Nuon Chea has always denied any guilt, saying he was not a cruel man, but "a patriot".
Nuon Chea is the second senior Khmer Rouge official to appeal his detention, following a similar move - also rejected - by Kang Kek Iew, AKA "Duch", who headed the group's Tuol Sleng prison and interrogation centre.
The panel of Cambodian and international judges, known as the Extraordinary Chambers in the Courts of Cambodia, is due to begin formal trials later this year.
However, earlier this month tribunal officials warned that a massive shortfall in funding could bring the entire Khmer Rouge trial process to a halt.
Many Cambodian legal staff working for the court have been told they could be made redundant within weeks unless extra funds are made available by the UN.
Tribunal officials have asked for a tripling of the court's original budget from $56m to $170m, saying initial estimates of the work involved fell woefully short.
But with allegations of corruption and poor management, there are doubts that the full request will be met.
The tribunal process has been blighted by delays and critics say they fear the few surviving members of the Khmer Rouge leadership may die before ever being brought to justice.