Nigel Farage, the leader of the populist Reform UK party, said the two men had held discussions about him joining the party.
Jenrick lost to Badenoch in the 2024 contest to lead the main opposition party after their crushing general election defeat and was then, in an effort to reunite the party, given the role of justice spokesman.
Jenrick has used that position to build a personal profile on key issues like immigration and crime that many saw as a platform for a future challenge to Badenoch's leadership, as the Conservatives sought to counter a dramatic loss of support to Reform UK.
"I was presented with clear, irrefutable evidence that he was plotting in secret to defect in a way designed to be as damaging as possible to his shadow cabinet colleagues and the wider Conservative Party," Badenoch said on X.
She did not say which party he was planning to defect to, and Jenrick did not respond to a request for comment.
Farage said he had previously spoken with Jenrick and had "little doubt" that he was considering defecting to Reform, although an agreement was not imminent.
Were that to occur, Jenrick would join a string of senior Conservative Party figures to switch allegiance to join Reform, which holds a solid lead in the polls.
"I'm very surprised that this news has broken," Farage told reporters in Scotland.
"Was I on the verge of signing a document with him? No. But have we had conversations? Yes."
Jenrick, who held various ministerial roles in the previous Conservative government, has undergone a transformation from being a staunch party centrist to moving towards the right with his strong criticism of immigration policies.
At least a dozen prominent Conservatives have joined Reform, although Jenrick would be only the second sitting MP to have switched to Farage's party.
Current opinion polling puts Reform ahead of both Prime Minister Keir Starmer's Labour Party and Badenoch's Conservatives.
That points to a possible 2029 Reform election win which would make Farage prime minister - a potentially seismic shift in a political system dominated by Labour and the Conservatives for more than a century.
The Conservatives are the United Kingdom's oldest and most successful political party, governing for 32 of the last 46 years.
But their reputation has been badly damaged by a 14-year spell in power that included a divisive Brexit referendum, several chaotic leadership changes and market crises, culminating in the party's worst electoral defeat in its history.
