Mr Niwatthamrong confirmed his participation in the meeting set for Wednesday, and hoped that plans for the general election would proceed smoothly.
The prime minister expected progress in the Wednesday meeting and insisted that he was authorised to prepare and forward a draft royal decree on the new election to His Majesty the King for endorsement.
"Progress at the meeting on May 14 will be a good sign for elections, and for the country to have a new government,'' he told reporters.
The government fully supports an election, "as soon as possible", with cooperation from authorities including the armed forces, he added. The July 20 date has been tentatively set by the EC.
With his status as acting caretaker prime minister, government opponents have questioned whether Mr Niwatthamrong has full, legal authority over the draft royal decree.
The EC insisted on Monday that Mr Niwatthamrong must attend the Wednesday meeting.
Pheu Thai Party members have poured scorn on the EC saying it it was reluctant to hold a new election. The last polls on Feb 2 were nullified by the Constitutional Court because protesters disrupted voting in some constituencies.