Election laws require a poll to be held between six and eight weeks from dissolution, and official sources say the date will most likely be 8 April, with parliament convening on 22 April.
The election commissioner must confirm the date of the election.
The BBC's Charles Haviland in Colombo says President Rajapaksa is hoping to capitalise on his presidential election victory in January by increasing the narrow majority of seats held in parliament by his Freedom Alliance.
Mr Rajapaksa fought the presidential poll on the back of the government's successful military campaign to end the long insurgency by Tamil Tiger rebels in the north and east of the country.
Sarath Fonseka had been planning to take part in the elections
Gen Fonseka also claimed credit for ending the civil war last year.
They fell out soon after the fighting ended and fought a bitter election campaign.
He was discussing the parliamentary election with supporters at the time. They say the arrest is an attempt to prevent his involvement.
Although the opposition had rallied around the candidacy of Gen Fonseka, our correspondent Charles Haviland says it is not clear whether the dour general could really have continued to bring the opposition together.
He says its fractious group of parties was already breaking up and forming new sub-alliances.
But our correspondent says it is not impossible for the general to take part in the parliamentary contest while in detention.
Mr Rajapaksa's alliance will be hoping for an absolute, two-thirds majority in parliament to cement his grip on power.
Anoma Fonseka: "They treated him as an animal"
In the last election in 2004, the Freedom Alliance won 105 of the 225 seats in parliament and the main opposition United National Party 82.
However, the Freedom Alliance has been able to maintain a workable majority with defections.
The Sri Lankan defence ministry says Gen Fonseka was arrested for breaking military regulations by holding discussions with politicians while serving on the national security council.
Gen Fonseka's wife Anoma says she has not been allowed to see her husband since the arrest.
Military spokesman Maj Gen Prasad Samarasinghe said Gen Fonseka was permitted to see family members and had been allowed legal advice.
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