Pro-Israeli organisations in Britain look set to see their influence increase if the Conservatives win the next election, a film scrutinising the activities of a powerful but little-known lobby warns today.

At least half of the shadow cabinet are members of the Conservative Friends of Israel (CFI), according to a Dispatches programme being screened on Channel 4. The programme-makers describe the CFI as "beyond doubt the most well- connected and probably the best funded of all Westminster lobbying groups".

Inside Britain's Israel Lobby claims that donations to the Conservative party "from all CFI members and their businesses add up to well over £10m over the last eight years". CFI has disputed the figure and called the film "deeply flawed".

The programme also describes how David Cameron allegedly accepted a £15,000 donation from Poju Zabludowicz, a Finnish billionaire who chairs Bicom (the Britain Israel Communications and Research Centre). Zabludowizc, the film reveals, has business interests in an illegal West Bank settlement. He also gave £50,000 to Conservative Central Office. Zabludowicz says his contributions "are a matter of public record".

William Hague allegedly accepted personal donations from CFI board members totalling tens of thousands of pounds after being appointed shadow foreign secretary. More than £30,000 from CFI supporters went to the campaign funds of members of Cameron's team who were first elected in 2005, the film claims, using publicly available information.

The programme-makers say that while this is legal, it is not well-known.

The CFI director, Stuart Polak, told the Guardian the figure of more than £10m is not supported by any facts. "It is fictitious, misleading and damaging to the reputation of CFI and its supporters," he said.

"CFI as an organisation has donated only £30,000 since 2005. Each of these donations has been made transparently and publicly registered. In addition to this £30,000, it is undoubtedly the case that some of our supporters have also chosen, separately, to donate to the party as individuals."

Two years ago a controversial study by American academics Stephen Walt and John Mearsheimer explored the influence of the Israel lobby over US foreign policy. But Britain's pro-Israel organisations have been subjected to far less scrutiny.

"The pro-Israel lobby … is the most powerful political lobby," Michael Mates, a Conservative MP and privy councillor, told the film-makers. "There's nothing to touch them."

Hague fell out with CFI after describing Israel's 2006 attack on Lebanon – in retaliation for a Hezbollah raid – as "disproportionate" and allegedly faced threats to withdraw funding from Lord Kalms, a major Tory donor and CFI member, the film reports.

Cameron later gave an undertaking not to use the word again, the programme claims. At a CFI dinner this June the party leader made no mention of the death toll in the Gaza war – 1,370 Palestinians and 13 Israelis. Instead he commended Israel because "it strives to protect innocent life".

Sir Richard Dalton, a former British diplomat who served as consul-general in Jerusalem and ambassador to Libya and Iran, said: "I don't believe, and I don't think anybody else believes these contributions come with no strings attached."

Labour Friends of Israel, another key group, is described as being "less unquestioning in its support of the Israeli government than CFI". But it has taken more MPs on free trips to Israel than any other group – more than 60 since 2001.

CFI has also flown over 30 Tory parliamentary candidates to Israel on free trips in the last three years.

Dispatches describes how when the presenter Jonathan Dimbleby criticised a pro-Israel campaign against the BBC's Middle East editor, Jeremy Bowen, Dimbleby was the subject of a complaint and, according to the programme, is now under investigation by the BBC. This point was denied in a statement later from the BBC's press office: "Any suggestion that Jonathan Dimbleby is the subject of an investigation by the BBC is incorrect."

Bicom, like the party-affiliated groups, organises briefings and trips to Israel for journalists, including Guardian staff. It sought to dismiss the significance of Zabludowicz's interest in a shopping mall in Ma'aleh Adumim, a settlement built on territory occupied in the 1967 war and which Israel would hope to retain.

Binyamin Netanyahu, Israel's prime minister, has rebuffed demands by Barack Obama for a settlement freeze.

Bicom's chief executive, Lorna Fitzsimons, said: "The private business interests of any of our funders – including our chairman – have absolutely no impact on Bicom's work.

"We are an independent organisation and we guard our reputation fiercely. We work with journalists to help them better understand the Middle East.

"We show Israel, warts and all, from the left to the right and we have a strict policy that on every journalist trip we go to the Palestinian Authority to give journalists unfettered access to Palestinian voices."

• This article was updated on 16 November 2009 to include a BBC denial issued later in the day about Dispatches' assertion that Jonathan Dimbleby was under investigation.