ABUJA: Nigeria's four main opposition parties have announced a merger, forming a coalition which could pose the biggest threat to President Goodluck Jonathan's ruling party since the end of military rule in 1999.

Previous attempts at opposition alliances have fallen apart due to infighting and regional differences. The president's party is likely to now target any weaknesses in the merger.

If the newly created party holds together it will be sternly tested when trying to agree on a presidential candidate for the 2015 election.

"At no time in our national life has radical change become more urgent," said a statement signed on Wednesday by representatives of the four parties.

"We, the following progressive political parties, namely ACN, ANPP, APGA and CPC, have resolved to merge forthwith and become the All Progressive Congress and offer to our beleaguered people a recipe for peace and prosperity."

The People's Democratic Party (PDP) has won every presidential election since a return to civilian rule 14 years ago. It controls around two-thirds of the states and has a healthy majority in both houses of the national assembly.

The four merging parties control almost all the remaining seats and marginally reduced the PDP's majority in both the states and parliament in elections in 2011.

"All the main opposition leaders have for some time backed the concept of a merger; the challenge will be to translate such common purpose from principle to practice," said Nigeria analyst Antony Goldman, head of Africa-focused PM Consulting.

"Policy and ideology do not feature prominently in a political discourse in Nigeria that is principally about winning or losing - and personal rivalries," Goldman said.

The PDP gave a confident response to the announcement.

"Beautiful. The more the merrier," the party's national chairman Bamanga Tukur told reporters.

"They are not a threat at all ... PDP is Messi in that contest," likening his party to the all-conquering Argentina and Barcelona soccer player Lionel Messi.

Tightly fought elections in Africa's biggest oil producer can often stoke violence. Hundreds were killed in riots in the mostly Muslim north when Christian southerner Jonathan won the presidential vote two years ago.

The merger, two years before the vote, will turn politician's attention to elections and away from vital reforms needed to support Africa's second largest economy.

It also mounts pressure on Jonathan to rally support from within his party and outside if he is to win re-election.

It is yet to be seen if every state governor and lawmaker supports the merger. Official changes of party will have to be agreed with the Independent National Electoral Commission (INEC). The INEC spokesman did not respond to calls for comment.