About half of Labour's candidates selected to fight in marginal seats at the next election have links to Westminster as former special advisers, party workers, researchers, lobbyists or MPs, research by the Guardian reveals.

The initial findings of a new project to map the backgrounds of candidates for 2015 suggests Labour and Liberal Democrats are choosing more from the political classes than the Conservatives, whose contenders are overwhelmingly male and drawn from business and finance.

Peter Hain, the former Labour cabinet minister who led a review of the party's relationship with voters, warned that traditional politics was now in a "state of terminal decline" fuelled by a sense of disconnection with the electorate. He said it was a problem across Europe that the political class was "reproducing itself" and urged local parties to speed up their attempts to engage with a wider range of people.

Fears of a widening chasm between the major parties and the electorate have intensified since the success of Ukip in the European elections. A Guardian/ICM poll has revealed 56% of people think the biggest problem with Westminster is politicians is breaking promises, while 44% are fed up with careerist MPs who "look and sound the same".

The Guardian examined the backgrounds of candidates in the most marginal seats – those that would require a swing of less than 5% of take a seat – as well as those with retiring MPs of the same party. There were 177 in total whose published CVs and other publicly available biographical information was then assessed.

In the 90 constituencies identified where Labour is the challenging party, the majority of candidates have some links to Westminster or Brussels in their background, including former or current staff of Ed Miliband, Yvette Cooper, Tessa Jowell, Ruth Kelly, Hazel Blears, Alistair Darling, Hilary Armstrong, Mary Creagh, Lord Foulkes, David Blunkett and Lord Sugar. There are also 15 Labour ex-MPs standing again, having already been ousted once by the electorate – one in five of those fighting the most marginal seats. These include some who were heavily criticised over their expenses arrangements before the last election, such as Joan Ryan and Andrew Dismore.

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Many Labour candidates with links to Westminster have done jobs in the outside world as well. For example, one prominent candidate, Mari Williams, standing in the target seat of Cardiff North, has been a deputy headteacher, as well as working as a political lobbyist and volunteering for Labour party HQ.

However, others appear to have worked for the most part in politics, including the rising star 31-year-old Tulip Siddiq, standing in Hampstead and Kilburn, the seat of the retiring Labour MP Glenda Jackson. Her CV includes being policy adviser to Jowell, deputy field director for Miliband's leadership campaign, press officer for London Labour, a researcher for Philip Gould Associates, a researcher for the Greater London Authority, a caseworker for the MP Harry Cohen and a job at Amnesty International.

Four Labour candidates in winnable seats are also related to former politicians. These are Will Straw, the son of the former home secretary Jack Straw, Stephen Kinnock, the son of the former Labour leader Neil Kinnock, Peter Keith, the husband of the former MP Shona McIsaac, and Richard Burgon, the nephew of the former MP Colin Burgon.

The Conservatives have so far selected fewer candidates for 2015 than Labour, but of the 52 marginals where they would need a swing of less than 5% to win or have a retiring MP, about one in five candidates has a link to Westminster politics.

The majority have backgrounds in business and finance, with a number of lawyers, military officers and private sector PR consultants also standing for office. In contrast to Labour, which has selected more women than men, fewer than a third of the Conservative party's candidates in these seats are female.

None of the three parties have so far selected many people from non-professional or blue collar jobs, with some exceptions being Lee Sherriff, a former retail worker in Carlisle for Labour, and Scott Mann, a postman standing for the Conservatives in North Cornwall.

Just under half of the 35 Lib Dems selected so far in the most winnable seats are being contested by people with links to Westminster or Brussels politics. These include Julia Goldsworthy, the former MP and special adviser to Danny Alexander, in Camborne and Redruth, and Ibrahim Taguri, the party's chief fundraiser, in Sarah Teather's seat of Brent Central.

The phenomenon of former Westminster workers becoming politicians is not new but appears to be flourishing the most within Labour, according to Dr Peter Allen, a lecturer at Bath University, who has studied the background of parliamentary candidates. "This is what has happened with Labour over the last 20 to 30 years," he said. "They increasingly seem to be picking people who have been political workers. It's partly that they used to get a lot more people from trade unions – who brought in traditional working-class occupations such as miners. The Tories are not as bad for this but they tend to take their candidates from business or the law and their supporters do not seem to be so bothered by the backgrounds. It is more a Labour problem."

Allen said the vast majority of sitting MPs … were in parliament.

"In terms of who runs the parties, it is these guys – and they mostly are guys," he said. "You can't underestimate the symbolic importance of who is in power because people will take their cues about what politics is about and like from what they see. If you see a lot of people who have nothing to do with your day-to-day life, you're going to begin to think politics isn't necessarily something for people like you."

Sharon Hodgson, the shadow equalities minister, said Labour was already doing a lot to increase its appeal through its Future Candidates programme, which helps give people from non-political backgrounds the skills to run for selection. However, she said there was "always more we can do" and the party was collecting information on the previous careers of candidates to see where barriers might exist.

She said many candidates who had worked in politics would have come from working-class backgrounds with a desire to fight for the party.

"Lots of people who are drawn to the Labour party are incredibly passionate about the causes that we fight for, and it's therefore no surprise that many of them want to work for the party or in other campaigning roles before running for office. That includes lots of people like myself from ordinary, working-class backgrounds," she said.

Hodgson, who left school at 16 and worked for almost two decades doing the books in the private sector and for a charity before going to work for the party and for Unison, stressed that a new focus on community organising and building a mass movement would change the party. This focus "should have the effect of bringing more people who have never thought about getting involved with politics into contact with the party", the shadow minister said.

Hain, who called in his Refounding Labour review for a looser political movement, rather than an inward-looking traditional party, said Miliband was the only party leader to grasp the scale of the challenge. But he argued that local parties should learn the lessons of seats such as Birmingham Edgbaston and Barking at the last election, where the MPs did well against the national trend by marshalling the support and interest of people who were not natural activists. "I still think the basic culture of the party has yet to change, frankly

I think Refounding Labour is being implemented but this is a much bigger change than the party has ever attempted since it was founded … It has still to be achieved outside a limited number of constituencies. It is a process that is ongoing. I actually think it is key to winning an election, to transform local parties, and the national party," he said

Hain also said there was a problem across Europe with professional politicians drawn from a class that was "reproducing itself", which was fuelling disillusionment with the system. "It is not the only factor but it means political parties are getting more distant from their electorate than they used to be," he said. "I think disconnection is so bad now that the old politics is in a terminal state. I think you can explain a lot of Ukip's rise and the anti-politics mood around that disconnection."

Some within Labour – from the former general secretary Peter Watt to the backbench critic John Mann – have advocated the more radical option of selection by US-style open primaries to bring in different candidates and reinvigorate new interest in the party. This would mean anyone in the constituency could help choose a candidate, which can help stop vested interests handing parachutes to their favoured choice. The Conservatives have experimented with this model in an effort to shake up the selection process but have restricted it to relatively safe seats over fears about their opponents trying to pack out meetings and swing the balance towards a weak candidate.

Criticism over Labour's selection has in the last year centred on unions getting their preferred candidates into top seats, which led to allegations of a stitch-up in Falkirk, the former seat of the retiring MP Eric Joyce. Around 17 out of 41 of Unite's preferred list of candidates, which was leaked last year, have now been selected – many of them in winnable seats. Of these, two are former MPs, three are lobbyists, four are full-time trade union organisers and two are trade union lawyers.

However, some on the left of the party are upset at what they see as an equally successful attempt by those on the Blairite Progress wing of the party to shoehorn preferred candidates into good seats.

Michael Meacher, former Labour minister, said unions remained the party's best hope of helping people from a diverse range of backgrounds into parliament, while candidates still needed contacts, financial backing and political knowhow to be successful.

: "Irrespective of right and left, there have been too many people who come through the traditional student politics, join the National Union of Students, get themselves a job as an assistant to an MP and the next thing is they are looking for a seat with the protection and support of the MP. I'm not saying that's wrong, it's a valid route, but it has been overdone. It does mean there are nothing like enough MPs who are working class. Everybody realises we need to rebalance the MP recruitment and do that in a way that is proportionate to class background. A lot of people see themselves as working class, expect to be represented as working class and at a time of six years of continuing austerity expect their MPs to be far more conscious of their plight than some of them are. The people most likely to do that are people who come from the same roots."

• This article was amended on 23 June 2014. The earlier version said Mari Williams had worked "for Labour party HQ". Williams' own website had said that she had "Worked for Tom Watson MP in Labour's HQ"; she has since asked us to clarify that this was a reference to "about a week" she spent volunteering for Watson at the party headquarters around 1998, and that she was not on the staff there.