David Cameron suffered a devastating double blow on the eve of the Tory party conference as his minister for civil society resigned over a sex scandal and a second Conservative MP defected to Ukip.

As the prime minister sent RAF fighter planes into action over Iraq, domestic troubles inside his own party dramatically punctured the optimism among Tory delegates as they headed to Birmingham for their last annual gathering before next year's general election.

The first blow came when Mark Reckless, the MP for Rochester and Strood, delivered the news that he was defecting to Ukip, at the anti-EU party's conference in Doncaster. His decision came just weeks after Douglas Carswell, formerly Tory MP for Clacton, had quit to join Nigel Farage's party saying the Conservatives were not serious about reforming the EU.

Blaming Cameron directly, Reckless, another Eurosceptic who wants the UK to leave the EU, said the Conservative leadership was "part of the problem that is holding our country back". Amid rapturous applause from Ukip delegates he announced that he would stand down in order to fight a byelection under Ukip's colours.

Voters, he declared, felt "ripped off and lied to" over Europe and immigration. They needed to believe that Britain had control over who came into the country and in what numbers. "At the moment we do not have any sense of that," he said. He added: "I promise to cut immigration while treating people fairly and humanely. I cannot keep that promise as a Conservative; I can keep it as Ukip."

Reckless won the seat for the Tories in 2010 with a majority of just under 10,000 over Labour. Farage told BBC News that he would do "whatever it takes" to get Reckless elected. "This man has shown huge courage. He has thrown his lot in with us and we will do everything we can to get him elected."

No sooner had the Conservative party absorbed news of Reckless's defection than it emerged that Brooks Newmark, a father of five and campaigner to increase the role and number of women in politics, had resigned from the government after being caught sending explicit pictures of himself over the internet to women, in a tabloid newspaper sting operation. After being told that the story would appear in a Sunday newspaper Newmark issued a statement saying he was quitting. He appealed for privacy for himself and his family.

Downing Street said his resignation had been accepted and that Newmark had been replaced by a former aide to chancellor George Osborne, Rob Wilson.

The double disaster means the Tories now meet amid a sense of crisis over the threat posed by Ukip, and sleaze, just at the time when they had hoped to unite and turn their fire on Labour. The party had been feeling more upbeat than for months, after Ed Miliband's Labour conference speech was heavily criticised in the media when it emerged that he had forgotten to make any direct mention of the deficit.

Reckless's shock decision to jump ship to Ukip means Cameron's party now faces two critical byelection fights against Ukip at the very time when it was hoping to concentrate on honing its anti-Labour messages ahead of next May's general election.

In a sign of concern among Tory moderates about the prospect of a shift to the right, former Home Office minister Damian Green warned Cameron: "We must at all costs and at all times resist the temptation to become UKIP-lite."

Sarah Wollaston, Conservative MP for Totnes, tweeted: "The Conservative party won't gain extra support by shifting right but millions of centre-right voters would move elsewhere, myself included."

She later added: "I'm staying put and fighting for the centre-right to be at the heart of the Conservative party, not a Ukip panic attack."

Michael Dugher, Labour's shadow minister for the Cabinet Office, described the defection as "a hammer blow to David Cameron's already weakened authority. David Cameron has always pandered to his right, and even they are now deserting him."

Tory chief whip Michael Gove held talks with Reckless after Carswell's defection as rumours circulated that he might be about to follow his close friend out of the party, but to no avail. After Carswell defected, Farage said that many more Tories would defect to Ukip if he held on to the seat in the byelection, an outcome that looks likely according to recent opinion polls. But he gave no hint that another defection might come so soon.

The Conservatives described the defection as "completely illogical". But it is bound to fuel speculation that other Tories could soon jump ship. As Reckless was introduced on stage at the Ukip conference by a clearly delighted Farage, the crowd broke out into whoops and cheers.

Asked about other possible defections, Farage replied: "We have these conversations. Of course, there are Conservatives I am talking to but there are Labour people too. There are Labour people who are deeply frustrated with Ed Miliband's leadership."

Tory veteran Michael Heseltine says that, while the Conservatives have to worry about Ukip, he believes Farage's party will be a "short-term phenomenon". The pro-EU former deputy prime minister says in an interview with the Observer that, rather than being deflected by Ukip, the Tories badly need to develop policies that resonate more strongly in deprived northern areas if they are to maximise their chances at the next general election. He sees the Tories' absence in large parts of Scotland and the north as a serious weakness the party must overcome.

Heseltine insists that, as the "rightful owners" of the one-nation mantra of government – and particularly in the light of the referendum on Scottish independence – the Conservatives must rebuild support in the poorest regions. With the party holding just one parliamentary seat in Scotland and increasingly absent across large swaths of the north, Heseltine describes the party's weakness in these areas as "deeply depressing".

"Without any shadow of doubt, there is a very important challenge for the Conservative party to have policies and to articulate policies in the language that resonates in those areas," he says.

He adds: "What is missing are the ladders of aspiration, trying to say to people in those communities, 'look, we can help you get out of this condition'. You may have to help them by knocking down a tenement block and building some decent housing, but they can't do that themselves.

"Just going on repairing the leaks is not dealing with the problem."

An Opinium/Observer poll puts Ukip on 17%. Labour on 34%, the Conservatives on 32%, the Lib Dems on 7% and the Greens on 4%.