* Case involves letters passed on by disgruntled tenant
* Britain says to raise matter with Pakistan government
* Blasphemy accusations on the rise, think-tank says (Adds British reaction)
By Katharine Houreld
ISLAMABAD, Jan 24 (Reuters) - Pakistan has handed a death sentence for blasphemy to a 69-year-old Briton with a history of mental illness, even though his lawyers were barred from the courtroom partway through the trial, the law firm said on Friday.
Accusations of blasphemy are surging in Muslim-majority Pakistan, according to an Islamabad-based think-tank, the Center for Research and Security Studies.
The charges are hard to fight because the law does not define what is blasphemous and presenting the evidence can sometimes be considered a new infringement. Many analysts see the allegations as score-settling or a front for property grabs.
Muhammad Asghar from Edinburgh was sentenced to death on Thursday, the law firm said, citing court officials in the city of Rawalpindi.
The firm said it was not present during the judgment because the judge had prevented it from representing Asghar in court since October.
The law firm asked not to be identified for fear of being targeted by extremists. Lawyers defending those accused of blasphemy frequently receive death threats and politicians supporting reform of the law have been killed.
In London, the British Foreign Office said it would be raising the matter urgently with the Pakistani government.
"We are aware that Muhammad Asghar, a British national, was yesterday sentenced to death by a court in Rawalpindi in Pakistan," it said.
"The Foreign and Commonwealth Office has been providing consular support to Mr Asghar, and we will be raising our concerns in the strongest possible terms with the Pakistani government."
Asghar was arrested in 2010 after writing letters to a lawyer and politician who said he was a prophet. Though Asghar did not post the letters, a disgruntled tenant whom he was in the process of evicting took them to police, the law firm said.
Asghar has previously been detained under the Mental Health Act in Britain and diagnosed with paranoid schizophrenia, it added.
The firm said after it was removed Asghar was given a state counsel, who did not put his medical history in evidence or call witnesses in his defence, and did not question a state-appointed board that declared him sane.
The state counsel could not be reached for comment.
A British doctor, 72-year-old Masood Ahmad, is also in prison in Pakistan, charged with blasphemy after a mullah used a mobile telephone to covertly record a conversation with him in his dispensary. (Editing by Alison Williams)