The Nigerian military issued its own statement, calling Mr Obasanjo, a former military ruler as well as an elected president, "a serious embarrassment" after he criticised its role in the postponement of the elections.

Our correspondent says the military's statement has raised concern among many Nigerians that it is taking sides in a political dispute rather than remaining neutral.

Prior to tearing up his membership, Mr Obasanjo told local media outside his residence that he was expecting to be ejected from the party.

"They said they want to expel me from PDP, although I have not been told, but I have my ears on ground," he said.

Mr Obasanjo said he would not be joining another party.

"I will only be a Nigerian, I'm ready to work with anybody regardless of political affiliation. Why would some people say they want to send me away, they don't need to bother themselves, here's your membership card, take it," he added.

Ex-military ruler Muhammadu Buhari is the the president's main challenge

Despite his pledge not to defect to another party, Mr Obasanjo has been vocal in his support for Mr Buhari, the candidate of the All Progressives Congress (APC), although he has fallen short of offering an outright endorsement.

Last week, Mr Obasanjo told London's Financial Times newspaper that he hoped the president was "not going for broke and saying: 'Either I have it or nobody has it'".

Mr Jonathan hit back at Mr Obasanjo, accusing him of threatening "national security" by whipping up opposition to his rule.

"It is very regrettable that in your letter, you seem to place sole responsibility for the ongoing intrigues and tensions in the PDP at my doorstep, and going on from that position, you direct all your appeals for a resolution at me," Mr Jonathan said.

Nigeria's election commission postponed the presidential and parliamentary election after the military said it would not be able to guarantee security at polling stations while it fought militant Islamist group Boko Haram in the north-east.

Mr Jonathan is contesting his second presidential election. His presidency has been marred by powerful PDP members defecting to the newly formed APC, fuelling speculation that he could be defeated in the election.

He has also been strongly criticised for failing to curb Boko Haram's insurgency.

Thousands of people have died as a result of the conflict over the past six years, while more than three million people have been forced from their homes.

The militants control a large stretch of land in north-eastern Nigeria and have stepped up attacks on neighbouring countries.