‘Yousef Abu Zuhri, 38, the brother of Hamas spokesman Sami Abu Zuhri, was martyred in an Egyptian prison as a result of torture,’ Fawzi Barhum, the other main spokesman for the group, told AFP news agency.
Egyptian officials have denied that torture was the cause of his death.
Yousef Abu Zuhri was arrested in the city of Al-Arish near Egypt’s border with Gaza in April on suspicion of entering the country illegally through an underground tunnel.
Sami Abu Zuhri said his brother died after weeks of ‘brutal torture in an Egyptian prison’ that led to severe bleeding.
He added that his brother was briefly treated at Egypt’s Alexandria University hospital but was returned to his cell before his condition had stabilized.
A spokesman for Egypt’s interior ministry said Abu Zuhri ‘died a natural death from a drop in blood circulation’.
AFP also quoted an Egyptian security official as saying on condition of anonymity that an autopsy said he died of natural causes.
Egypt has been repeatedly accused of torturing prisoners by former detainees and human rights groups.
Earlier this year, the Egyptian Organization for Human Rights said 13 citizens had been tortured to death between June 2008 and February 2009.