The guidance should silence primary care organisations (PCOs) who claimed that they did not know about the changes.
Dr Andrew Dearden, GPC negotiator with responsibility for pensions, said: 'It would appear that the Pensions Agency didn't tell PCOs about the changes.'
Last month the NHS Pensions Agency confirmed that GPs could shore up their income in the first month of retirement by working outside the NHS.
The new pension rules allow GPs to draw their full pension so long as they take a 24-hour break and then limit their service to an aggregate of 16 hours a week for the first month of retirement.
'Employment outside the NHS does not count towards the 16 hours,' the agency said.
Before the rule change, GPs were required to take a one-month break from working for the NHS.
The letter also confirmed that the 16-hour limit does not affect GPs who take early retirement due to ill health or redundancy.