Prime Minister Najib Abdul Razak claimed the opposition was trying to strain his relationship with Deputy Prime Minister Ahmad Zahid Hamidi.
He accused the opposition of being envious of the solidarity displayed in the Umno annual general assembly last week.
"Therefore, it is surprising that they are spreading rumours in an effort to spoil the close relationship I have with the deputy prime minister," he said in a blog posting today.
Najib did not provide specifics but it is believed the prime minister was responding to a "fake" letter in Zahid's name.
In the Dec 9 letter addressed to BN MPs and senators, it urged them to pressure Najib to step down as prime minister.
The letter listed the replacement cabinet with Zahid as prime minister and Khairy Jamaluddin as deputy prime minister.
Umno secretary-general Tengku Adnan Tengku Mansor earlier today dismissed the letter as a hoax.
"Maybe there are some parties that are uneasy with the close relationship between the prime minister and deputy prime minister.
"So they come out with all sorts of slander to divide them. Both leaders have no problem," he was quoted as saying.
In his blog post today, Najib expressed confidence in Zahid, stressing that he was unlike other Umno leaders who have defected.
"Perhaps the opposition made wrong assumptions about the character of the deputy prime minister by thinking that he is like a few former Umno leaders who were guided by personal interest instead of the party's and national interest.
"They have tried everything within their means to split us but they do not know the loyalty of Umno members," he said.
Najib said Umno members will remain loyal to the party and its democratically-elected leaders through thick and thin.
However, he cautioned Umno members against those trying to sow disunity.
"I urge all Umno members to be cautious in the face of the opponent's dirty tactics.
"They are jealous of the solid solidarity in Umno and BN," he said.
The prime minister also took a thinly veiled swipe at the opposition's cooperation with former prime minister Dr Mahathir Mohamad, who was Umno president for 22 years.
"The chaotic opposition is so desperate that they have been forced to ally themselves with a leader who previously had them jailed," he said.- Mkini