Nigel Farage suspects David Cameron raised the UK terrorism threat level in order to distract people from the fact Tory MP Douglas Carswell defected to Ukip.
Farage also suggested the prime minister had timed today's high-stakes Commons vote on British involvement in military action in Iraq to overshadow the Ukip conference in Doncaster.
"I think if this is that vital, why on earth didn't parliament meet yesterday? There's no earthly reason it couldn't meet yesterday," he told BBC Radio 4's Today programme.
Asked if he seriously believed the government had timed today's vote, which is expected to immediately trigger RAF airsrikes against Isis, was being done to take the spotlight of Ukip, Farage added: "The day after Douglas Carswell joined Ukip we were told Britain had been put on a heighten state of emergency, so listeners can make their own minds up."
On 28 August, Carswell surprised Westminster by announcing he was leaving the Conservative Party and joining Ukip. The switch, which had been a closely guarded secret, gave Farage wall-to-wall TV and newspaper coverage.
The following day the government raised the terror threat level from "substantial" to "severe", in response to the escalating crisis in Iraq and Syria. Cameron said the move had been made, in party, amid concerns about Britons travelling to the region to join extremist groups.
Farage is not the only leading Ukip politician to voice suspicion that Cameron scheduled a vote on Britain going to war to detract from the gathering of 2,000 Ukip supporters in the Midlands. "Just fancy that - Cam may recall Parly on 1st day of Ukip conference to announce that he has finally worked out which side to bomb in Syria," the party's communications director and MEP Patrick O'Flynn tweeted.