The EU referendum campaign has been a relentless battle of facts. Surveys and polling tell us which claims are resonating with voters. But are partial surveys influencing public opinion?

An undercover investigation by Channel 4 News tonight reveals evidence of surveys made from a secretive call centre that appear to be feeding negative messages stated as fact to voters – a practice experts say appears to be “push polling”.

Documents and secretly filmed footage obtained by this programme show a London-based company has been calling voters claiming to be conducting an “independent survey”.

Following up a source who initially approached news outlet The Canary, we went undercover – to find out what the firm were up to.

Voters are asked a series of questions including how they intend to vote on Thursday. Scripts to be read over the phone by callers and obtained by Channel 4 News show a series of assertions presented to voters as fact.

In one question, voters are apparently asked to grade issues by importance on a scale of one to five, and are then given a series of one or two word concerns, including “economy”, “employment” and “immigration”.

Among the options, the script to be read by the callers then states as fact: “The EU costs Britain £350 million per week, how important is the cost of the EU?”.

The claim is hugely controversial, and has been branded as both misleading and patently untrue for ignoring the rebate back to Britain and EU money pumped back into the UK.

Documents and undercover research by the Channel 4 News investigations team show call centre workers were ordered to say the poll was “for research” and “not biased”.

Another version of a script we obtained and to be read to voters states as fact: “The EU costs each family £700 per year. How important is the cost of EU?”

This statement is disputed by Remain campaigners who claim the figure, citing the London School of Economics, is that the EU saves families £350 each year.

On some calls, these suggestive and leading statements are only to be read out only to undecided voters.

The calls have been made for months from a secretive operation in a non-descript building in north London.

Inside is a call centre for Return Research, a marketing and market research business. Since April, and possibly earlier, a team of callers has been phoning voters about the referendum.

Based on the targets written on whiteboards inside the firm, Return Research is conducting up to 20 surveys per person per shift.

Polling experts say these calls could influence voters.

Andrew Hawkins, managing director of respected polling company ComRes, said some of the questions appear to be akin to push-polling, despite other more traditional questions.

“Where I think we stray from the straight and narrow is where claims are made which clearly are contested claims around Britain’s contribution to the EU and presented as fact.

“That would in my view doubtless fall foul of the Market Research Society’s own code of conduct and it would suggest that there is as much of an attempt to persuade the respondent with those questions as there is an attempt to find out what the respondent actually believes.”

Like push pulling?

“Yes,” Mr Hawkins said.

A Return Research supervisor admitted to us in a secretly-recorded interview that some surveys appear slanted: “There was a question in there, 1 to 5, and it was ‘The EU costs £350 million per week, how important is that to you?’ And obviously that’s what the Leave campaign keeps spreading out.”

So who’s funding the work? And has Return Research been conducting push-polling on behalf of one side of the EU referendum campaign?

Return Research said in a statement: “Return Research has not carried out any political campaigning in relation to the EU Referendum on Thursday. We carry out impartial research for our clients.”

Channel 4 News has learned that the firm is however conducting the work for a third-party. not political parties or either the Vote Leave or Remain campaign – but won’t say who it is.

When callers are suspicious and ask who is behind the survey, the call staff are told to say they don’t know the end client – “Who is it run for? I’m afraid I don’t actually know. I work for Return Research, we’re not told the end client in order that we and those we speak to remain impartial.”

If asked where the survey would be published, they were told to reply: “Not all the results we gather are published. It’s often purely for research purposes. They are often used for newspapers/TV polls.”

However, there is no evidence of any polls by Return Research being published, or used by the media.

Hotly-disputed claims like the costs of the EU seem to be stated as fact. So is this independent research or is it really political marketing? And who is behind it?

Confusingly, it is understood that some workers themselves have been told the client is the Remain campaign, others believe it to be the Leave side.

We asked Vote Leave if they had commissioned Return Research, were hiring them or working with them in any way – either directly or indirectly. Remain also said that they hadn’t commissioned the firm.

But they categorically denied using the company or its services. So did Leave.eu – the other big leave campaign, as well as Grassroots Out and Ukip.

It’s quite possible, of course, that it’s merely the work of a wealthy Brexit maverick.

A very wealthy one.

The Information Commissioner said it hasn’t yet had any complaints from the public about the firm – and will act if made aware on any concerns.

If you’ve been called by Return Research or have a story that we should investigate – email us at c4investigations@itn.co.uk.