An Egyptian Christian remains in prison for reporting attacks on churches, despite other journalists being pardoned.

Bishoy Boulos, a Christian convert, was not among several high profile journalists to be released by Egyptian President Abdel Fattah al-Sisi ahead of the current UN summit in New York. One hundred people were pardoned by al-Sisi but Boulos' ongoing detention has sparked concern among campaign groups.

"It's great news that high-profile journalists were set free," said Paul Robinson, the Chief Executive of Release International.

"But Bishoy Boulos remains in jail for reporting on attacks against Christians and for asserting his claim to be a Christian. Egypt must set Bishoy free and deliver on its promise to guarantee full religious freedom."

Boulos, also known by his former Muslim name, Mohamed Hegazy, was arrested in December 2013. He was accused of "harming the national interests of the state" by reporting the burning of churches and attacks against Christians.

However Release International, which describes itself as the voice of the persecuted church, claim his ongoing detention has more to do with his conversion from Islam to Christianity.

"Bishoy should have been released in May," a statement said. "But then a second charge was levelled against him of blasphemy against Islam. And that case is still under investigation."

The blasphemy charge is linked to his attempt to change his religious identity from Muslim to Christian on his official ID card, the campaign group said.

"Bishoy's continued detention probably has more to do with his earlier conversion from Islam than to any alleged offence connected with reporting," said Robinson.

Boulos first made international news in 2007 for being the first Egyptian Christian convert to file a lawsuit to try and change his religious status. Since then, he has been arrested, tortured and threatened with death, according to the statement.

"The continued persecution of Bishoy Boulous reveals two things,' said Robinson. "The authorities are reluctant to face the truth about continuing attacks against Christians, and converts to Christianity remain at serious risk despite the country's new constitution, which is supposed to protect religious freedom.

"Unless there is a clear mechanism to enforce the rights of the country's Christian minority, those rights are likely to remain every bit as theoretical as they have been in the past," he added.