The National and Islamic Forces, an umbrella organisation which brings together 13 different factions, said on Sunday the gathering will be an unwanted interference in Palestinian affairs.
According to reports, Blair had wanted to host a wide-ranging peace conference but was forced to scale down the meeting's agenda in the face of Israeli pressure.
The conference is now to focus on Palestinian political and economic reforms, and Israel has made clear it will stay away.
"Blair's proposal for the London conference, which was supposed to discuss the peace process, has been transformed into an interference in Palestinian internal affairs," the factions said.
The National and Islamic Forces include representatives of all
the main Palestinian factions, from the Fatah movement supported by the leadership to Islamist political organisations Hamas and Islamic Jihad.
The revised nature of the meeting means that it will serve
"American interests and the interests of [Israeli Prime Minister
Ariel] Sharon's government," said the factions.
"Blair's proposal for the London conference, which was supposed to discuss the peace process, has been transformed into an interference in Palestinian internal affairs"
Palestinian Liberation Organisation leader Mahmud Abbas, the favourite to win next month's presidential election, had welcomed the prospect of a conference after he held talks with Blair in the West Bank last week.
However, splits soon emerged among the leadership about the conference, with Prime Minister Ahmad Quraya taking umbrage at what he regards as arrogant British suggestions that the Palestinians need "grooming" in the peace process.
Senior Palestinian sources said that Quraya had been taken aback that Abbas had given his written support for the conference after other senior officials had voiced their reservations about its agenda.
Hamas has already urged the Palestinian leadership to boycott the talks.