Boris Johnson has warned potential rebel Tory MPs they would be "utterly nuts" to defect to Ukip.
The London Mayor launched a direct appeal to Eurosceptics as he insisted only the Conservatives would give voters a say in future membership of the European Union.
His intervention comes amid suggestions further MPs could be set to follow backbenchers Mark Reckless and Douglas Carswell in defecting to Ukip.
Mr Johnson dismissed those clams as "fanciful" - but issued a direct appeal to anyone considering following them to think again.
"Let me try to reason with any in my party, the folk who want to launch themselves headlong into the silage, because I genuinely think that they (if they indeed exist) must be utterly nuts," he wrote in his Daily Telegraph column.
"There are only two people who are in a position to take the keys to Downing Street next May, and they are David Cameron or Ed Miliband. I know that you disagree vehemently with most of what Ed Miliband says and does - and so I must urge you not to allow the disaster of a Miliband premiership.
"If you really want to let this country sleepwalk into a Labour government, then that is your prerogative. You can close your eyes and let it happen. You kip if you want to; the rest of us are going to fight and win."
His intervention came as Chancellor George Osborne tried to get the Birmingham conference back on track for his party as he announced the scrapping of the so-called "death tax" on pension pots.
The Mayor of London will be joining Amol Rajan on London Live this Thursday at 12.30pm. Put your questions to him by visiting londonlive.co.uk/askboris
