Former Olympics minister Dame Tessa Jowell has appealed for urgent cross-party talks to halt a decline in school sport, as an Observer investigation suggests the coalition is destroying hopes of a grassroots legacy from London 2012.

Exactly six months on from the start of the Games, an Opinium/Observer poll shows that only 10% of people believe the government has played its part in increasing support for sport in schools and communities in the runup to – and since – last summer's Olympics.

By contrast, 55% say support has either stayed the same or been cut back since the coalition came to power. Among parents of youngsters under 18, the figures are even more stark. Only 17% of parents say government support has increased since 2010, while 65% say it has either stayed the same or been cut.

At last summer's Games, Team GB won a record medal haul, inspiring hopes of a surge in sporting participation and government action to boost the legacy. Six months on, a damning 59% of parents say they have either seen a decline or no improvement in government support. Only 24% say support has increased.

London won the right to stage the 2012 Olympics after convincing the judges that the Games would "inspire a generation" of UK sportsmen and women, and by promising that a permanent legacy would be built in schools and communities. Today a separate Observer survey of the views of headteachers in the state sector shows that many believed the promise had been on target for delivery until 2010, but a large majority now think things have gone into sharp reverse since the education secretary, Michael Gove, began dismantling Labour's system of School Sports Partnerships (SSPs).

Chris Dunne, headteacher of Langdon Park school in Tower Hamlets, London, described the government's decision as "an enormously destructive act, verging on vandalism" and said it was a tragedy that sporting opportunities had been cut back dramatically either side of the Games.

David Ellis, head of York high school, said: "It really is a national disgrace that we are not building on the goodwill, enthusiasm, excitement and motivation that London 2012 generated."

Last night, Jowell, who persuaded sceptical Labour colleagues, including Tony Blair, to bid for the Olympics in 2002 and 2003 and was made a dame in the New Year's honours list for her work in delivering the Games, said a cross-party deal had to be struck if the legacy promises were to be met. Previous behind-the-scenes efforts by Jowell to bring parties together have failed.

"These findings are very worrying but not surprising. But it is not too late to rescue the legacy," she said. "This government has got itself into a terrible bind by shutting down a world-class system of School Sport Partnerships and now pride is the only thing that is stopping them putting things back on course.

"This really is urgent and I believe my Labour colleagues would be more than happy to sign up to a cross-party agreement that would find a solution."

Gove withdrew the £168m funds for the SSPs as one of his first acts in government. Under the SSPs, certain secondary schools were chosen as sports "hubs", which then co-ordinated expert teaching across all primary schools in their areas and organised after-school clubs. Gove also abolished the target of at least two hours of PE per pupil per week. Under Labour, the proportion of children reaching this target had risen to over 90%.

During the Olympics, David Cameron appointed Lord Coe as his legacy "ambassador" as concerns over the lack of a legacy in schools threatened to overshadow the Games. Since then, although there is support and money to provide more help for primary schools, arguments within government over whether this money should be "ringfenced" continue to hold up progress.

A spokesman for the Youth Sports Trust appeared to accept a decline in school activity. "It is, of course, disappointing to hear of any reduction in high-quality PE and school sport offered to our young people," said the spokesman. "However, we are aware of some excellent work being undertaken by headteachers and schools across the country to maintain a sporting culture in young people from the earliest possible age. But we also know that more can be done – and for some schools, especially primary schools, more support is required. We are therefore pleased that the government is considering a new approach to school sport and await their announcement with interest."

A long-awaited report by Ofsted into levels of sport in schools over the past four years, which is expected to show a fall, is now expected early next month after being repeatedly postponed.

Labour has promised to restore the minimum two-hour target and to ensure Ofsted carries out regular inspections. While unwilling to commit to restoring all previous funding, it says it would ensure that schools join partnerships.

Stephen Twigg, Labour's shadow education secretary, said: "Six months on, the Olympic legacy is in tatters. We've seen partnerships that enabled primary schoolchildren to do competitive sport scrapped and the number of pupils doing two hours of sport a week collapse from nine in 10 under Labour to half now.

"Labour would get Ofsted to inspect school sport, enable teaching assistants to become PE teachers, and ensure parents know how much sport their children get at school. Headteachers say the Labour government was making progress, but it's been lost."

A government spokesman said: "We are working on a variety of measures to improve school sport as part of the Olympic and Paralympic legacy. We will announce our plans in due course."