This newspaper revealed earlier this week that a programme to upgrade outdated sewerage systems at some 60 pollution "hot spot" locations has slipped significantly.

Concerns about other areas have now been raised following the disclosure of a high-level Government memo from 2004.

It stated that the list of hot spots was in danger of growing, with further sewage treatment facilities becoming overloaded through housing growth.

The memo was written in January 2004 by the DoE's top official, Permanent Secretary Stephen Peover, to Minister Angela Smith. The hot spots total at that stage stood at 57.

Mr Peover briefed the minister on the DoE's controversial policy of permitting ongoing housing development in these areas. His memo stated: " There are now other locations (in addition to the 57 hotspot locations) where the waste water infrastructure is now overloaded; there are others where the current situation is acceptable, but increased loading from future development might cause them also to become hotspots. They are being internally labelled 'Warm Spots'."

Mr Peover's January 2004 document informed the minister that planning applications for more than 25,000 new houses had been submitted for the 57 hotspots since October 2002 - the date the DoE lifted a development freeze in these areas.

Northern Ireland is at serious risk of daily fines from Europe for failing to comply with EU standards on sewage treatment.

The hotspots upgrade schedule was meant to be completed this year, but has since been extended to at least 2009.

The January 2004 memo was obtained by Friends Of The Earth under environmental information legislation.

Spokesperson for the campaign group Declan Allison said: "This DoE memo raises more questions than it answers. Where are the so-called 'warm spots' and what progress has been made in ensuring they don't become polluting 'hot spots'?

"The memo said the situation would be under continual review and would be 'open and transparent'... In that spirit of openness and transparency, Government should now answer these important questions."

A spokesman for the DoE's Environment and Heritage Service (EHS) said: " The term 'warm spot' is used as a figure of speech. There are none as such.

"Any location where the infrastructure can currently cope could, in theory, become overloaded if significant development occurs - housing or industrial - without any necessary upgrades in infrastructure occurring to accommodate the development.

"Where EHS considers that development may cause infrastructure to become overloaded we alert Planning Service as part of the normal process."