President Mahinda Rajapakse on Wednesday retained the defence portfolio as required by the constitution and also kept the finance ministry, as most of his predecessors have done.
Sripathy Sooriyarachchi, made minister of employment, quit soon after being sworn in, saying the non-cabinet rank job he was given was beneath his qualifications and he was also protesting the exclusion of Marxists from the cabinet.
No posts were given to the Marxist JVP (People's Liberation Front), or to the all-monks JHU - the National Heritage Party. The two nationalist parties were key allies of Rajapakse in last Thursday's election which he narrowly won.
The JVP quit the previous government in June after falling out with the then-president Chandrika Kumaratunga over proposals to share tsunami aid with Tamil Tiger rebels. It supported Rajapakse's bid for president.
Political sources said squabbling for top jobs in the cabinet had delayed the swearing in of the new government, originally scheduled for Monday to coincide with the induction of Prime Minister Ratnasiri Wickremanayake.
The appointment of Wickremanayake raised concern among some analysts about the prospects of a ceasefire in place since 2002 with separatist Tamil Tiger rebels.
The ethnic war has claimed more than 60,000 lives since 1972.
Wickremanayake (R) was
sworn in as prime minister
Sooriyarachchi criticised the president for keeping the JVP out of the cabinet although they had backed him.
"I am sad about the JVP that was in the forefront of the presidential campaign not getting powerful cabinet portfolios," Sooriyarachchi said after sending his resignation letter to the president.
"This is not in keeping with my qualification," he added. "I'd rather continue my law practice."
Sooriyarachchi was a key spokesman for Rajapakse's election campaign.
"What this (cabinet) signals is an early parliamentary election," said a minister who would not be named.
"The president can improve the strength of the SLFP (his moderate Sri Lanka Freedom Party) and reduce the dependency on the JVP."
The all-monks party said it had not expected any cabinet posts but would support Rajapakse in parliament.
"This is not in keeping with my qualification. I'd rather continue my law practice"
The JHU insists that Rajapakse drop plans for a federal state in exchange for ethnic peace in the troubled island where more than 190 people were killed this year in violence linked to the conflict despite a 2002 ceasefire.
There was no immediate reaction from the JVP about being left out, but a party statement said it was calling a press conference "to appraise the decisive steps the JVP will be taking under the present political situation".
A top government source said the JVP had been offered five portfolios, but there had been disagreement and the party eventually declared it would not take up any positions.