* Jazeera reporter held without charge since last year
* Started hunger strike in protest in January
* Jazeera says he's being kept in solitary confinement
* Calls on world powers to secure his release
By Andrew Osborn
LONDON, May 20 (Reuters) - Qatar-based satellite network Al Jazeera has written to world powers asking them to secure the release of one of its journalists jailed in Egypt, accusing the authorities in Cairo of endangering his life.
In a letter seen by Reuters, a lawyer acting for the pan-Arab network said the health of Abdullah Elshamy, one of four Al Jazeera reporters being held in Egypt, was "of the gravest possible concern and in need of immediate attention".
Elshamy, who has been on hunger strike since Jan. 21 to protest against his detention, is being held in solitary confinement in dire conditions, the letter said.
"Mr Elshamy's situation is of grave concern; his health is deteriorating and the Egyptian authorities show no sign of providing appropriate medical care or of bringing an end to his entirely unwarranted and indefensible detention without charge," Cameron Doley, a lawyer acting for Al Jazeera, wrote.
"Time is of the essence."
Al Jazeera's intervention, designed to put pressure on former Egyptian army chief Abdel Fattah al-Sisi ahead of a May 26-27 presidential election which he is expected to win, is likely to further sour already poor Qatari-Egyptian relations.
Elshamy, who is Egyptian, was arrested in Cairo in August last year while reporting on police dispersing supporters of ousted Islamist president Mohamed Mursi, toppled the previous month by Sisi.
Qatar, a Gulf Arab monarchy that funds Al Jazeera, backs Mursi's deposed Muslim Brotherhood, which Cairo has declared a "terrorist" group. Qatari ties with Egypt have been strained since the army ousted Mursi after mass unrest against his rule.
Doley, a lawyer for London law firm Carter Ruck, said in the letter that Elshamy had acute anaemia, the onset of kidney dysfunction, low blood pressure and hypoglycaemia, and that his weight had dropped from 108 to 68 kilogrammes.
Recipients of the letter included U.S. Secretary of State John Kerry, the European Union's foreign policy chief Catherine Ashton, British Foreign Secretary William Hague, French Foreign Minister Laurent Fabius, and U.N. human rights chief Navi Pillay.
Al Jazeera last month served Egypt with a $150 million compensation claim for what it said was damage to its media business inflicted by Cairo's military-backed rulers.
Three other Al Jazeera journalists are being tried in Egypt on charges of aiding members of a "terrorist organisation", in a case that human rights groups say shows the authorities are trampling on freedom of expression.
All three deny the charges and Al Jazeera has said the accusations are absurd. Egyptian officials have said the case is not linked to freedom of expression and that the journalists raised suspicions by operating without proper accreditation.
The trio - Peter Greste, an Australian, Mohamed Fahmy, a Canadian-Egyptian national, and Baher Mohamed, an Egyptian - were detained in Cairo on Dec. 29.
Earlier this year, an Egyptian prosecutor said Al Jazeera journalists had published lies harming the national interest and had supplied money, equipment and information to 16 Egyptians. The foreigners were also accused of using unlicensed broadcasting equipment.
Both state and private Egyptian media have fanned anti-Brotherhood sentiment, suggesting anyone associated with the veteran movement is a traitor and threat to national security.
Egyptians often ask journalists in the streets whether they work for Al Jazeera. Saying yes could mean a beating.
The Brotherhood renounced violence as a means of political change decades ago and says it remains committed to peaceful activism, denying any association with the surge in Islamist insurgent violence since Mursi's downfall.
The crackdown on dissent has raised questions about Egypt's democratic credentials three years after an uprising toppled veteran autocratic president Hosni Mubarak and raised hopes of greater freedoms. Mursi won power in a free election in 2012. (Editing by Raissa Kasolowsky)