A 17-year-old boy has admitted dropping flammable material on to the M3, causing it to be shut for 11 hours.
He pleaded guilty to arson recklessly endangering life after a substance was dropped from a road bridge.
The motorway shutdown caused gridlock on 23 September. The substance was later discovered to be flammable but not explosive.
The teenager, who cannot be named for legal reasons, will be sentenced on 2 March following psychiatric reports.
He also pleaded guilty at Winchester Crown Court to eight counts of burglary at Winchester College, two counts of blackmailing the headmaster and two of theft.
He pleaded not guilty to two counts of causing a danger to road users by obstructing the road. The court heard those charges will lie on file.
The discovery of a partly-ignited substance, dropped from a bridge on Saturday 23 September, led to military bomb disposal experts being called to the motorway near Winchester.
Thousands of motorists, including football fans travelling to the Southampton v Manchester United match, were caught up in queues up to three miles long when the road was shut between junctions nine and 11.
On 16 September there was a similar incident on the same bridge, which leads to St Catherine's Hill.
Seven of the burglary charges related to thefts of computer equipment at top public school Winchester College between August 2016 and June 2017.
The court heard the teenager twice demanded payment of £10,000 in the online currency Bitcoin from the school in order for him to stop carrying out burglaries.