At times the journal reads like that of any other young person in a foreign land, homesick and unhappy. But according to Pakistani intelligence officials this journal is the diary of a British jihadist - a 25-year-old from the London suburbs who intended to give up his life in a holy war.
Mr Siddique, who spent a number of years in Hounslow, was arrested in May by the Pakistani security forces after they apparently received reports that he was acting suspiciously. When the authorities entered his room in the north-eastern city of Peshawar, investigators apparently found an electrical circuit that could be used as a bomb detonator and a desktop computer that contained aeronautical mapping and the cryptic 35-page diary written in English and covering the period from 2 March to 6 April this year.
Across the top of the first page of the diary was a quotation from the Koran. It read: "The greatest tests are truly to be soon alleviated." In Mr Siddique's first entry he wrote: "All alone in a strange land. I can trust no one except Allah." In an entry on 26 March he questioned how fellow Muslims could live peacefully in London when the "kufr" (unbelievers) have turned the world into a "battlefield for the Muslims". London, he wrote, was the "vital organ of the minions of the devil". On 5 April he said he would make an "all-out immense effort to rejoin my contingent".
The New York Times reported that Pakistani officials believed Mr Siddique was preparing to become a suicide bomber. They said phone numbers found with Mr Siddique have been traced to members of al-Qa'ida as well as British extremists involved in a failed plot to detonate bombs in London in 2004. These reports cannot be confirmed.
It was also reported that British police are investigating whether Mr Siddique has ties to the July 7 attacks. They want to know whether a diary entry on 13 March in which Mr Siddique says he has learnt that "wagon is now called off" refers to the bombing plot.
Officials said that Mr Siddique, currently in custody, has denied having any role in the failed 2004 plot or the recent attacks in London. He has apparently told them that he first travelled to Pakistan in February 2003 with another British Muslim - one of eight men later arrested on suspicion of involvement in the failed 2004 plot. He said he spent three months in Lahore with Mohammed Junaid Babar, a Pakistani-American computer programmer from New York. Last year Babar pleaded guilty to charges of supplying military equipment to an al-Qa'ida training camp in Pakistan.
Officials said he had also told them that he spent the past two years fighting in Kashmir and Afghanistan. At one point he railed in his diary against those Pakistanis who "claim 2 b Muslim" but "don't get it thru there thik [sic] heads" that it is their "fard" - or religious duty - to help him wage his holy war.
His diary is written in mostly capital letters and reveals how he quickly grew uncomfortable with those around him in Peshawar. "I can't live in filth unlike u animals," he wrote on 8 March, calling a group of Pakistani neighbours "dirty geezers" and a Pakistani store owner a "monkey con artist".
Two days later he complained that he could not converse with the locals. "[I'm] constantly laughed at & ridiculed," he wrote.
In an entry on 11 March he wrote about his visit to a group of people he identifies with code names and learnt of bad news. "The relaxing place was done over and 7-8 of the guys taken whilst asleep. Told guys need 2 make a move soon. Cant stik round." Several days later, on 15 March he was told that the "situation is really bad" and that he should "just sit tight & wait it out until things get a bit better".
Mr Siddique has told investigators that he is from Hounslow and is a Muslim of Indian descent. A local newspaper reported that in 1997 the then 17-year-old "ran off to join the mujahedin" in Lebanon but returned to his "frantic parents" a month later.
Officials in Hounslow said last month that Mr Siddique attended a college where he may have befriended Asif Muhammad Hanif, who blew himself up in the suicide bombing of a Tel Aviv nightclub in 2003.