Every year a very grand lunch is given by the Conservative Friends of Israel at a central London hotel. Anyone who is anyone in the Conservative party makes it their business to be there. It is normally addressed by the party leader.

This year's event took place in June, with the main speech by David Cameron, and the shadow foreign secretary, William Hague, in attendance. The dominant event of the previous 12 months had been the Israeli invasion of Gaza. We were shocked Cameron made no reference in his speech to the massive destruction it caused, or the 1,370 deaths that resulted, or for that matter the invasion itself. Indeed, our likely future prime minister went out of his way to praise Israel because it "strives to protect innocent life". This remark was not intended satirically.

Afterwards, we resolved to ask the question: what are the rules of British political behaviour that cause the Tory leader,his mass of MPs and parliamentary candidates to flock to the Friends of Israel lunch in the year of the Gaza invasion? And what are the rules of media discourse that ensure such an event passes without even being noticed?

During an investigation lasting several months, we have been able to reach several important conclusions. We maintain there is indeed a pro-Israel lobby in Britain. It is extremely well-connected and well-funded, and works through all the main political parties.

For instance, Conservative Friends of Israel (CFI) – once described by the famous Conservative politician and historian as "the largest organisation in western Europe dedicated to the cause of the people of Israel" – claims that 80% of all Tory MPs are members. The Labour Friends of Israel is equally formidable. In 2001, Jon Mendelsohn, a former chairman of LFI and now Gordon Brown's chief election fundraiser, was quoted in the Jerusalem Post as saying that "Zionism is pervasive in New Labour. It is automatic that Blair will come to Labour Friends of Israel meetings."

Since 2001, LFI has arranged more than 60 free trips to Israel for MPs. LFI and CFI trips account for an astonishing 13% of all funded trips abroad for MPs and candidates. That's more trips to Israel, with a population smaller than London's, than to any other country.

Furthermore, those in many sensitive foreign affairs, defence and intelligence posts in the Commons are often Labour or Conservative Friends of Israel. Mike Gapes, chair of the foreign affairs select committee, is a former deputy chair of the LFI. Kim Howells, chair of the intelligence and security committee (and a former Middle East minister), used to chair LFI. James Arbuthnot, chair of the powerful Commons defence select committee, is also the serving parliamentary chair of the CFI.

The Israel lobby is not afraid to use its political muscle. After Hague said Israel's invasion of Lebanon in 2006 was "disproportionate", there was an explosion among donors. Only a few weeks ago Hague, following CFI pressure, put out a statement demanding Britain reject the Goldstone resolution at the UN.

Meanwhile, a parallel operation is carried on against media organisations that criticise Israel's foreign policy. In particular, the Guardian and the BBC suffer from a barrage of complaints and emails, many from outside the UK. The BBC has proved unable to cope. As the culture secretary, Ben Bradshaw (a former BBC reporter), rather bravely remarked after director general Mark Thompson turned down a request from the Disasters Emergency Committee to broadcast a humanitarian appeal for Gaza: "I'm afraid the BBC has to stand up to the Israeli authorities occasionally. Israel has a long reputation for bullying the BBC."

It is important to say what we did not find. There is no conspiracy, and nothing resembling a conspiracy.

Yet, as we demonstrate in Dispatches on Monday night, the financial arrangements of a number of the organisations that form part of the pro-Israel lobby are by no means widely known. The pro-Israel lobby, in common with other lobbies, has every right to operate and indeed to flourish in Britain. But it needs to be far more open about how it is funded and what it does. This is partly because the present obscurity surrounding it can, paradoxically, give rise to conspiracy theories that have no basis in fact. But it is mainly because politics in a democracy should never take place behind closed doors. It should be out in the open for all to see.