Opinion polls have shown that such a grouping could defeat the ruling Institutional Revolutionary Party (PRI), even though none of the eight parties would stand a chance by itself.

The PRI has faced growing opposition recently

The agreement on an agenda for the formation of the Alliance for Mexico came after weeks of negotiations between the eight parties, which include the conservative National Action Party (PAN) and the centre-left Party of the Democratic Revolution (PRD) that dominate Congress.

The party leaders insisted in their statement that their goal was not solely to defeat the PRI in next July's polls.

They said the object was to "take advantage of the historic opportunity'' to conclude Mexico's transition from a virtual one-party state to democracy.

The parties have so far agreed on a seven-point plan in preparation for forming a coalition. This includes the selection of a joint presidential candidate by holding some form of "national consultation".

But correspondents say there are still substantial ideological differences and personal rivalries to be overcome before the parties formalise the coalition.

Cronyism

The ruling PRI this week launched its first presidential campaign since it took power in 1929.

Four candidates are expected to compete for the party's nomination in primary elections set for 7 November.

The primary will be the first in the history of the PRI. In the past, ruling presidents have always chosen the candidate for the succession.

The PRI has faced growing opposition over recent years. It has been dogged by accusations of corruption, cronyism, mismanagement and even suspicions that some top members may have been involved in political assassinations.