Wales could be facing a rubbish crisis as tips which do not meet EU standards are forced to close by 2007, the Environment Agency Wales has warned.
The Swansea landfill site is due to close
It told the assembly's environment committee there needs to be an "urgent" review of waste disposal plans.
In October, the National Audit Office for Wales said the nation would run out of waste disposal sites by 2010.
But a review of eight of 22 landfill sites has already found four are not up to standard and will have to close.
Nicholas Parr, the agency's policy advisor for waste regulation, told BBC Wales' news website all 22 landfill sites for household waste had to apply for permits.
These, he said, would not be renewed if they failed to meet the standards in the EU's landfill directive.
Since October, the agency has reviewed eight sites, and found four were not up to the required standards.
One was Swansea, which will have to send its waste to other parts of Wales when its tip closes within the next month or two.
Mr Parr said each site was judged in its own right, and the closure of four did not mean the same proportion would go on to fail inspections.
He added: "It's early days and it's small numbers, but we wanted to flag that there is an issue here.
"Even if [just] some sites close, it is going to bring the date forward from 2010."
He said the agency wanted to stress that alternative ways of dealing with waste needed to be found, such as composting, to divert waste away from landfill site.
Recycling must also play a major role.
The agency has urged Wales' three regional waste plans to be "urgently reviewed".
In north Wales, an joint Environment Agency, police and council initiative has started using digital technology to capture people who fly tip on camera and bring them before the courts.
They will put up signs similar to police speed camera signs warning the public that illegal dumping of waste may not go unnoticed.