The weapons were being moved between Birmingham and London along the M40 when the convoy was forced to stop on the carriageway.
An escort vehicle travelling with the convoy, which was carrying four nuclear warheads, was forced to stop on the hard shoulder of the M40 near junction nine near Bicester at around 11am on Monday.
Campaign group Nukewatch said the transport signalled a "disturbing disregard for public safety".
A convoy carrying nuclear warheads broke down on the M40 near Bicester
The Ministry of Defence (MoD) said it operates the highest standards "in accordance with stringent safety regulations”.
The incident caused travel chaos for drivers with slip roads around the area being closed and traffic on the northbound carriageway grinding to a halt.
Reports claim the convoy was travelling from the warhead assembly plant at Burghfield to Coulport on Loch Long, where the warheads are stored and loaded onto the Trident submarines.
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The convoy eventually moved off again at 11.45am, followed by a truck which had arrived on the scene to tow the escort vehicle.
A spokesperson for Nukewatch, which monitors nuclear arms around the country, said: “The breakdown happened to an escort vehicle rather than one of the warhead carrying trucks but clearly any vehicle can have problems.
“After all there have been 180 incidents involving the convoys in the period 2000 to 2016.
The convoy had to stop on the M40 when one vehicle broke down
The UK’s Ministry of Defence is allowed to carry explosive and radioactive materials in the same transport – a procedure rationally outlawed for civil transport.
“It is probably only a matter of time before we have a serious accident involving these convoys.
“The transport of these weapons shows a disturbing disregard for public safety, which is put in second place by the government’s determination to continue to deploy these horrific weapons of mass destruction.”
The explosives boast an impact range of 600 metres, according to the MoD and the radiological plume would extend to at least 5 kilometres (3.1 miles).