Four men were on Wednesday sentenced to life imprisonment for conspiring in connection with the failed attempts to bomb London's [Images] public transport system in July 2005, while the remaining two will face re-trial after a jury failed to reach a verdict.

Judge Adrian Fulford said all the four convicts must spend a minimum of 40 years in prison before becoming eligible for parole.

The four -- Muktar Said Ibrahim, Yassin Omar, Ramzi Mohammed and Hussain Osman -- were found guilty on Monday over a failed attempt to set off four bombs in the city on July 21, 2005, two weeks after suicide bombers killed 52 commuters.

The judge at Woolwich Crown Court in south-east London said that both the July 7 bombings and the July 21 failed attacks were carried out by terrorist cells under the control of Al Qaeda [Images].

He also ordered that the remaining two -- Manfo Kwaku Asiedu (34) and Adel Yahya (24) -- would face a retrial over their involvement in the plot.

A date for the retrial is expected to be set by the end of this month.

Though all the six defendants denied the charges saying that devices were duds and their actions were a protest against the Iraq war, police said that scientific tests proved the bombs were all viable.