IMR is a convention, rahter than being a statue law passed by parliamenet that is binding.
All government minsters, incluing the prime minster, are drawn from parliament and are therefore accountable to it.
This is both in relation their responsibility for the performance of their department there integriy and personal conduct
Responsibility for department
The current minsiterial code sets out that ministers have a duty to parliament to account and be held to account, for the politics, decisions and actions of their departments and agencies
The idea that government minsters are responsibile for the running, policies and performance of their departments and can be held to account by parliament for this is key to individual ministerial responsibility.
Responsibility for department
Expected to accept responsibility for any failures or criticisms of their department. Any key failures should lead to a resignation.
The current ministerial code also sets out 'it is of paramount importance that ministers give accurate and truthful information to parliament, correting any inadvertent error at the earliest opportunity.
Responsibility for personal conduct
Ministers are also responsible for their personal conduct and professionalism. If their personal conduct falls below what is expected of them, they are expected to offer their resignation. This is a far more common cause of minsiterial resignations than departmental failures.
How is it enforced?
They are supposed to be advised by their independent advisor, it is ultimately the prime minister that enforces the ministerial code
They are the 'judge, jury and executioner' and the enforcement of individual minsterial responsiblity is therefore entirely dependent on the prime minister
As we have seen with party gate, there is no mechanism to hold the PM themselves to account when they are accused of wrongdoing.
Complications and recent changes to individual ministerial responsibility
Civild servants being blamed - the lines of accountability have been blurred and civil servants have been held accountable for failings within departments, rahter than government ministers. - Traditionally, civil servants were anonymous and took no credit or blame for the actions of government
March 2023 - Suella sent an email to Conservative party supporters blaming an activists blob of leftwing lawyers and the labour party for the failure of her department to stop channel crossings
Following the failure of the exams algorithm in the summer of 2020, head of ofqualsally Collier resigned, having overseen the development of the algorithm, whilst secretary of state for education at the time Gavin williamson didnt resign.
Executive agencies - under director generals rahter than under direct control of government ministers. These include the prison service, court service and DVLA.
As a consequence, tehre is doubt over who should be held accountable for failures with the ministers responsible for the overall policy area, by the direct generals exervising 'operational responsibility'
1995 the home secretary Michael Howard controversially sacked Derek Lewis, the director general of the prison service, after critiscim of the escape of prisoners from Parkhurst Jail.
Boris Johnson changes to ministerial code in may 2022
After the partygate scandal broke out and dominated the news, Boris Johsnon revised the ministerial code
There is alot of suspicion Johnson did so to prevent himself and many of his ministers having to resign over party fate, as he and other misled parliament.
The key change johnson made was to weaken the rules on individual ministerial responsibility so that ministers who breach the ministeiral code are no longer expected to resignm but to publicly apologise and or accpet a reduction in their pay.