Junead Ahmed Khan and his younger 22-year-old uncle Shazib Ahmed Khan were also charged with planning to join Islamic State militants in Syria, the Crown Prosecution Service (CPS) said.
The preparation for the alleged attack is said to have taken place between May and mid-July.
No other details were immediately available.
The two men are accused of planning their trip to Syria between August 2014 and May 2015.
“It is alleged that Junead Khan and Shazib Khan had been planning on travelling to Syria to join the proscribed organisation Islamic State in Levant (ISIL),” said Deborah Walsh, deputy head of counter-terrorism at the CPS.
“It is further alleged that Junead Khan was planning a terrorist attack on US military personnel in the UK and he has been charged with an additional terrorism offence to reflect this.”
The two men were arrested on July 14, along with another man in his 30s, who was later released without charge.
“The arrests and searches were part of an ongoing investigation into Islamist-related terrorism,” police said. – AFP