They're less prepared than their peers were in 2006, when the last survey was carried out.
Researchers said incidents including the Manchester Arena bombing and the Grenfell Tower fire in London exemplify the need for a major incident plan.
Experts at the University of Oxford wanted to find out if the relatively low levels of preparedness more than a decade ago had changed and so conducted a phone survey.
They contacted 296 specialist trainees (on-call registrars) in emergency medicine, trauma and orthopaedics, anaesthetics and general surgery from 74 hospital trusts that had dealt with more than 30,000 patients in emergency care in the first three months of 2017.
They had responses from 186 (63%), while the rest either did not respond or did not consent to being included in the survey.
Since 2004, all hospitals in England have been required to keep a major incident plan, or MIP for short, to respond to an incident involving a large number of casualties.
Half the specialist on-call registrar respondents (50%) had not read the plan at all, while around one in four (25%) had read only part of it.
Less than half the respondents (47%) knew where to find a copy of the plan, one in six (just under 18%) definitely did not know where to locate it, while the rest were unsure.
When asked what role they would have in a major incident response, just over a third (36%) knew what they would be required to do.
A similar proportion (37%) were not sure, and around one in four (27%) did not know.
Among those responding, emergency medicine doctors were the most prepared, but the fact that the overall situation has not changed in 12 years is worrying, researchers said.
"Of concern, we found no improvement since 2006: indeed fewer individuals were confident in the role they would play if an MIP came into effect while they were on call," their report says.
The report is called Preparation For The Next Major Incident: Are We Ready? A 12-year update.
Primary author, Dr Jamie Mawhinney, now at St Thomas' Hospital, said: "The take-home message from my study is that the majority of middle-grade doctors in England are not confident in the role that they should play in a major incident.
"We were surprised that this was the case, especially given recent high-profile emergencies in the UK such as the Grenfell Tower disaster and the London Bridge and Westminster terror attacks.
"Our results in fact show that registrar doctors are less confident in responding to the major incident plan than previously.
"In order to improve confidence amongst staff I believe it will be necessary to increase training.
"Specifically we believe that all doctors should receive education on their hospital's major incident plan at all trust inductions, as well as an abbreviated version of their own particular role."
The researchers pointed out that this is an observational study, and the findings need to be taken in light of the response rate of just under 63%.
They added that the study included only specialist registrars who would currently be expected to lead the emergency response, but major trauma centres in the UK are moving towards 24/7 consultant cover.
A spokeswoman for the NHS said: "This survey, based on 0.2% of all hospital doctors, flies in the face of the experience of the hundreds of people who have received expert, quick and professional care in response to incidents like the attacks on Manchester arena, Westminster and London Bridge.
"But the NHS is not complacent and training exercises and reviews of previous incidents are carried out regularly to maintain the highest possible levels of preparedness."