Haroon Syed, who was in court, pleaded guilty at the Old Bailey court in London to "researching, planning and attempting to source" materials for homemade explosive devices for use in the attacks.
Syed, warned by the judge that a life sentence is likely to be handed down, will be sentenced in June after a probation and psychiatric report.
The Londoner pleaded guilty to a plot running from April to September 2016 to get materials for a bomb to stage attacks.
He was arrested in September 2016 after he told an undercover officer of his desire to get bomb-making material and chatted online with undercover British agents.