There is uncertainty in Tonga over the government's commitment to political reform by 2008.
Last Thursday, the pro-democracy campaigners claimed that the prime minister's letter confirmed that 21 members of the 30-strong assembly would be elected by the people.
The concession was made in the face of popular protests outside the assembly building shortly as riots were about to erupt.
The government had earlier said changes could not be instituted before 2009 at the earliest.
Our correspondent Mateni Tapueluelu, says the concession appears invalid.
"The committee of the people have had a second look at that particular letter that was signed by the prime minister. It doesn't seem to have some weight, some political weight."