David Cameron is guilty of "pandering to prejudice, uncertainty and anger" as he responds to the threat posed by the UK Independence party by championing a "negative and uninspiring" form of politics, a leading moderniser has warned.

Ryan Shorthouse, the head of the Bright Blue thinktank, warns No 10 that it cannot "outdo Ukip" – echoing deep concerns among modernising ministers that the prime minister has abandoned his early optimistic approach, which won him the Tory leadership in 2005 and unsettled the Labour party. They fear the change of stance is damaging the Conservatives.

Shorthouse said: "At the moment, the messaging is quite negative and uninspiring – it's not enough to win voters and gain momentum. We need to be more inspiring and bigger picture than that and we need a positive vision, not just pandering to prejudice and uncertainty and anger.

"There has been a surge in Ukip, but you can't outdo Ukip. I often think the more you make an issue out of things like immigration and benefits, the more you make an issue out of things like benefit tourism, people will think it is a really big problem and Ukip are the ones that benefit."

The remarks by Shorthouse, who used to work for the culture secretary, Maria Miller, and the universities minister, David Willetts, echo the fears of ministers and modernising officials who have recently left Downing Street.

There are particular concerns that Andrew Cooper, the prime minister's director of political strategy, who urged a reluctant Cameron to embrace modernisation a decade ago, was sidelined after the arrival of Lynton Crosby as the Tories' general election campaign manager. Cooper, who is a member of Bright Blue's advisory board, recently left No 10 to return to the Populus polling company he founded.

Shorthouse, who is close to Cooper, expressed alarm at the recent focus by No 10 on cracking down on benefits."Every time I read the papers at the moment it's a clampdown – someone from the No 10 policy board has been talking about reducing benefits for people with more than two children and then there is clamping down on new immigrants," he said. "It is a very unbalanced negative message and we didn't come into politics to clamp down on vulnerable people and benefit claimants, for instance, or to lay traps for Labour."

The Bright Blue director said he understood the need to keep tabs on welfare bills and to manage immigration. But he warned that the relentless focus on immigrants risked sending the wrong signal about the Tories.

Shorthouse said: "A lot of people think that immigrants come here, work hard, contribute – which the overwhelming majority do – and that's a good thing. People's aspirations make the world go round and Tories should be behind aspirational people. The Tories need to make sure that, yes, people are concerned about public services and that they are managed properly.

"But they also need a very positive story to tell about immigration. For example, higher education, is a world class export for Britain – four of our universities are in the top 10 world universities. We should be attracting students to come."

The intervention by Shorthouse follows the decision by Tory modernisers to confront Cameron in private last month after a minister told the Daily Mail and the Sun that the prime minister wanted to "get rid of all this green crap".

Adding his voice to the calls, Lord Howe of Aberavon, who served as chancellor and foreign secretary in Margaret Thatcher's government, said Cameron had made his position more difficult by his "repeated concessions" to the Eurosceptics.

He said Cameron must give a lead on the issue and not leave it to outside individuals and organisations – from US president Barack Obama to the CBI.

"The more people understand and experience the reality of Britain's place in today's interdependent world – an interdependence which Conservative policies in the 1980s did so much to promote – the less likely they are to want this country to leave the European Union," Howe said.

The group, which included of ministers, ex former ministers and parliamentary aides were alarmed that the prime minister was abandoning his optimistic "Vote Blue, Go Green" approach.

One Tory at the meeting later told the Guardian: "The agenda is not being junked. It just feels undernourished."

Shorthouse lamented the change in tone from 2005: "There's a generation of people in their 20s and 30s who came to work for the party under Cameron and campaigned for it just after 2005, because of the more liberal Conservatism. It was new and inspiring, there was something magnetic about it that really attracted a new generation of people, there was a real buzz around it. A lot of people believe it was just a PR exercise and vacuous triangulation but actually there are deep roots to it; it wasn't just rebranding. Of course there was an image problem and the Tories needed to address that, but there were deep roots."

Shorthouse said Tory modernisation was based on three core principles: supporting people in poverty but in ways that went "beyond mere cash transfers"; creating a Conservatism that was more human and "went beyond Jaguars and spreadsheets"; and abandoning dogmatic ideology to be open to new ideas.

The modernisers should be proud that they introduced policies such as the green deal, the renewable heating incentive and the troubled families programme, he said.

"A lot of people who were involved in that early modernisation have now left Downing Street," Shorthouse said. "I fear, and I think they do too, that the messaging is becoming more narrow and a lot of MPs have talked about this as well, as things have changed at the top there has been a narrowing of the message."

But Shorthouse believes the modernisers made mistakes by allowing themselves to be depicted as obsessed with metropolitan issues. "We're now being pigeonholed into just caring about and being obsessed with metropolitan issues like gay marriage and green issues. But it was much deeper than that. Actually a lot of modernisers are from different, quite ordinary backgrounds and we need to make sure their voices, their faces are being put out there.

"I think some people think the modernising stuff, the Big Society, childhood, is all fluffy. It isn't, it actually means a lot to people and it is incredibly of deep concern to people – things around the safety of their children, the opportunities for their children. There is this idea that it is just a bunch of liberal-minded, fluffy-minded people, but it isn't – it actually speaks to what people really deeply care about, and that's what we're in politics for: not to spread prejudice, but to really help people and change their lives. It should be a positive – the reason we are in politics is positive, inspiring messages and policies, not the narrow politicking and flaying Labour. We should try and go beyond that."

Bright Blue will publish a "liberal Conservative manifesto" in April, to ensure there is a "balanced, moderate message" from the Conservatives. This will call for an increase in the minimum wage and a focus on schools, childcare and dementia – issues that matter to the vast majority of people.

"It's time to capture that optimism and openmindedness again," Shorthouse said. "Often those of a moderate disposition are fearful of putting their head above the parapet, but we must provide energy behind that modernising agenda and make sure that No 10 hears there are actually quite a lot of us in the party and around the party who want that agenda to carry on."