Manfo Kwaku Asiedu, 34, had been due to stand trial for a second time next week on the more serious charge of conspiracy to murder. He pleaded guilty to the lesser charge during a pretrial hearing at the Central Criminal Court.
He is due to be sentenced Nov. 19.
A jury convicted four men of murder conspiracy in July, but was unable to agree on verdicts for two other defendants - Asiedu and Adel Yahya, 24.
The charges stemmed from failed attempts to detonate explosives on London subway trains and a bus on July 21, 2005 - just two weeks after suicide bombers killed themselves and 52 commuters in the city.
At the earlier trial, Muktar Said Ibrahim, Yassin Omar, Ramzi Mohammed and Hussain Osman were sentenced to minimum terms of 40 years in prison after being convicted of conspiracy to murder.
All six defendants had denied the charges, saying the devices were duds and their actions a protest against the Iraq war. But police and prosecutors said scientific tests proved the bombs, made from a volatile mix of hydrogen peroxide and four, were all viable. They do not know why they failed to explode.
Ibrahim, Omar, Mohammed and Osman attempted to set off their bombs on three subway trains and a bus, in an echo of the July 7 attacks, but the main charges did not detonate and no one was hurt.
Prosecutors said Asiedu lost his nerve and abandoned his device in a London park. It was shown to the jury as evidence.
During the trial, Asiedu turned on the others and claimed Ibrahim, the gang's self-proclaimed leader, had wanted the attacks "to be bigger and better" than the July 7 bombs.
Yahya, who had left Britain for Ethiopia several weeks before the attacks, was sentenced Monday to six years and nine months in prison after admitting collecting information likely to be useful to terrorists.