He will carry out his cabinet shuffle at 11 a.m. ET at Rideau Hall.
"[It] will be a substantial and significant shuffle," a source told CBC's Evan Solomon, host of Power & Politics and The House. "There will be new faces and younger members alongside experienced and capable hands."
The source also said there will be new women in cabinet and that the government's focus will remain on the economy.
Finance Minister Jim Flaherty is keeping his job and Foreign Affairs Minister John Baird is also keeping his portfolio, Solomon reported.
But at least three ministries are guaranteed to have new people in charge. Harper has to fill the vacancy left by Vic Toews, who quit as public safety minister and as the MP for Provencher in Manitoba last week.
Keith Ashfield, the current fisheries minister, has cancer and asked the prime minister to relieve him of his duties in the next shuffle. The intergovernmental affairs department lost Peter Penashue as the minister in March when he resigned over inappropriate election claims. He was defeated in the byelection and Transport Minister Denis Lebel assumed the intergovernmental affairs job.
Lebel now has his hands full with the train disaster in Lac-Mégantic.
Environment Minister Peter Kent has made it known he won't mind if he is bumped out of cabinet and junior finance minister Ted Menzies and Diane Ablonczy, minister of state for foreign affairs, aren't running again in the 2015 election which means they could be dropped.
With the Senate expense scandal still hanging over the Conservatives and declining support in the polls, political experts and insiders say this shuffle is a crucial one for Harper.
Some say this is an opportunity for Harper to try and change the channel by putting a fresh face on his cabinet. They also expect this shuffle to be coming ahead of a throne speech in the fall when Harper will lay out new policies and priorities for his government.