"People must be aware of this move. There are federal plans in this to divide the country. The place of Buddhism also is to be diluted," Rajapaksa told reporters in the central district town of Pilimatalawa.
The government led by Rajapaksa's successor Maithripala Sirisena has set in motion a process to formulate a brand new constitution replacing the current 1978 constitution.
The Tamil minority urges a political power devolution to the north and east in the new constitution.
His comments came a day after the Chief Minister of the Tamil-dominated Northern Province C V Wigneswaran held a protest rally in Jaffna demanding a federal solution to the minority's aspirations for political recognition.
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The rally advocated a merged north and east province as the federal Tamil state.
Rajapaksa as President in 2006 had taken action through a court order to separate the two provinces - which had been merged in 1987 in terms of the Indo-Lanka Peace Accord of the then Indian Prime Minister Rajiv Gandhi and the then Sri Lankan President J R Jayewardene.
Reacting to Wigneswaran's demands, Deputy Minister of Defence Ruwan Wijewardena said the government would ignore all demands even as it grapples with the issue of achieving reconciliation with the Tamils.