A 19-year-old man has been jailed for life for planning a bomb attack that may have targeted an Elton John concert or Oxford Street in central London.
Haroon Syed, of west London, admitted preparing acts of terrorism after trying to source weapons including a suicide bomb and machine gun.
He was caught after approaching MI5 officers, who were posing as a fellow extremist, via social media.
Syed was sentenced to a minimum of 16 years and six months.
Last year, his brother was jailed for life for plotting to behead someone on Remembrance Sunday.
Judge Michael Topolski QC said the risk Syed posed warranted a discretionary life sentence.
'Do martyrdom'
Prosecutors say Syed's plans ranged from becoming a suicide bomber to staging a gun attack, and while he initially boasted of working with others, those people did not materialise.
Instead, over the summer of last year, he made increasingly urgent efforts to secure weaponry.
After he went online looking for help, a purported jihadist fighting overseas, known only as Abu Isa, introduced him to another extremist going by the name Abu Yusuf.
This second man was, in fact, a group of MI5 officers who were playing the role of a jihadist in what became weeks of social media chat with Syed.
Duncan Penny QC, prosecuting, told the Old Bailey there was initially some "suspicion on both sides" before Abu Yusuf concluded that Syed was a "committed brother" he could deal with.
Syed then began talking about his aspirations and gave his contact a shopping list, saying he wanted "do martyrdom" after first causing "damage" with a machine gun.
"Can you get the gear?" asked Syed. "You will be involved right?
"Two things. Number one, machine gun and we need someone who can make a vest you know the dugma [button] one. So after some damage with machine gun then do itishadi [martyrdom] ... that's what I'm planning to do."
The undercover officer told Syed guns were expensive - but he might be able to get someone to build a bomb. Syed floated the idea of going to fight overseas with his new-found friend - but revealed his passport had been cancelled by the authorities.
He tried and failed twice to get fraudulent loans of thousands of pounds to cover the cost of firearms - and eventually agreed to meet his contact in a coffee shop in Slough, Berkshire, to finalise an alternative plan.