The guidance emerged just days after a CCG in the south-west of England, hailed as a beacon of integrated care, revealed it was seeking legal advice over fears about the impact of competition rules.
Ministers also faced continuing opposition to Health and Social Care Act legislation in the House of Lords this month. Former health secretary Lord Owen introduced an 'NHS Bill' calling for the controversial section 75 of the Act to be dropped.
The draft guidance from Monitor aims to help commissioners 'comply with new procurement, patient choice and competition regulations'.
It claims that 'choice, competition and integrated care are not mutually exclusive'.
But it goes on to say anti-competitive behaviour by commissioners may be allowed if they can demonstrate that it is in patients' interests.
The draft guidance says: 'Where a commissioner's conduct is in the interests of patients, its behaviour will not be inconsistent with the prohibition on anti-competitive behaviour.'
A decision not to open services up to competition may be acceptable if it leads to 'services being provided in a more integrated way', co-operation between providers to improve patient care, or if it helps to increase 'the number of patients treated by a provider where higher patient volumes result in better outcomes'.
Monitor chief executive David Bennett said: 'The guidance makes it clear that the regulations do not force commissioners to go out to tender for every service, but equally commissioners should not simply roll over existing contracts without first asking how good the service is and whether it could be improved to give patients a better deal.'
The guidance also suggests CCGs could handle conflicts of interest in commissioning decisions by drafting in GPs from neighbouring areas or asking health and wellbeing boards to oversee their work.
A spokeswoman for South Devon and Torbay CCG confirmed it had sought legal advice over the extent to which it needed to open up services to competition.
She said: 'We have received some legal advice, but in the meantime we are getting on with our plans for further integration.'
Lord Owen's Bill calls for commissioners to be free to buy whichever services are best and to 'face no legal obligation to foster markets'.