Cabinet Office minister Brooks Newmark resigned after reportedly sending explicit pictures of himself to an undercover newspaper reporter in a tabloid sting.
His announcement came just hours after Rochester and Strood MP Mark Reckless sent shock waves through the Tory ranks with his declaration that he was joining Nigel Farage’s “people’s army”.
He is the second Conservative to defect to Ukip within a month, joining Clacton MP Douglas Carswell.
For the Tories gathering in Birmingham for the final time before the general election in May, there could hardly have been a worse start to their annual conference.
According to the Sunday Mirror, Mr Newmark - the Minister for Civil Society and a founder of the Women2Win campaign group - allegedly exchanged X-rated pictures over the internet with a reporter posing as a Tory PR girl.
The 56-year-old married father of five tendered his resignation after learning that the newspaper was about to publish details of their exchanges.
“I have decided to resign as Minister for Civil Society having been notified of a story to be published in a Sunday newspaper. I would like to appeal for the privacy of my family to be respected at this time,” he said in a statement.
Mr Cameron, arriving last night in Birmingham with his wife Samantha, wished waiting reporters “good evening” but did not respond to questions about the twin setbacks.
Nevertheless there was deep anger in the Conservative ranks, with a party spokesman denouncing the move as “completely illogical”, warning “a vote for Ukip is a vote for Ed Miliband” and Labour.
His constituency party chairman, Andrew Mackness, said that he was “astonished and disgusted” by the MP’s action.
“Only 48 hours ago he proclaimed his support for the Conservatives and their plans for a referendum on Europe and he gave me assurances he wouldn’t defect,” he said.
“He has misled the hard-working people of Rochester and Strood who voted for him.”
The Tories will seek to deflect attention from the resignations by setting out plans for a fresh cap on benefits in order to fund millions of new apprenticeships if they win next year’s general election.
Chancellor George Osborne said a Conservative government would cut the maximum level of benefits a household can claim from £26,000-a-year to £23,000.
At the same time unemployed young people aged 18 to 21 would be given six months to find work or training - after which their dole payments would be withdrawn unless they agreed to take take part in “community projects” such as cleaning up local parks.
Most unemployed 18 to 21-year-olds would also be prevented from claiming housing benefit in order to leave home under the Tory proposals.
In an interview with The Mail on Sunday, Mr Osborne said the changes would pay for 3 million apprenticeships over the lifetime of the next parliament.
“Our mission is not just to save the pounds here and there, we’re trying to change the welfare system so it doesn’t trap people in poverty and a culture of dependency. It is a tragedy for them and a waste for the country,” he said.
“It is not acceptable for young people under the age of 21 to go straight from school on to benefits and into a home paid for through housing benefit - benefit funded by other people who are working.
“We are saying you will receive an allowance but if you can’t find work after six months, you will have to work for the dole. They are difficult decisions but the right ones.”
The Tories believe benefit cuts are popular with voters - although they are likely to be strongly criticised by anti-poverty campaigners.
The Conservatives have already tried to reduce the benefits cap in government but have been blocked by their Liberal Democrat coalition partners.
“Before our reforms, some families were receiving £100,000 a year in housing benefit. How many working people can afford rent of £100,000? It was a gross injustice. Since we imposed a cap, large numbers have looked for work,” Mr Osborne said.