David Cameron, pictured on the steps of Downing Street with his wife Samantha returning to No 10 following the shock election win, was said by pollsters to be in a dead heat with Ed Miliband

Pollsters may have deliberately tweaked their surveys to ensure they produced similar results to their competitors ahead of last year's election - meaning they were all wrong.

The preliminary report of an official inquiry into the polling disaster, which saw every pollster forecast a dead heat when in fact the Tories won outright, said it could not rule out 'herding'.

The inquiry said the finding did not necessarily mean any polling company had done anything wrong but said 'design decisions' in how each firm produced its results nudged all results toward a consensus.

The report found: 'A surprising feature of the 2015 election was the lack of variability across the polls in estimates of the difference in the Labour and Conservative vote shares.

'Having considered the available evidence, the Inquiry has been unable to rule out the possibility that ‘herding’ – whereby pollsters made design decisions that caused their polls to vary less than expected given their sample sizes – played a part in forming the statistical consensus.

'It is important to note that the possibility that herding took place need not imply malpractice on the part of polling organisations.'

Polling is a lucrative business, particularly in an election year, and firms are regularly scrutinised to see which produced the best forecast.

The across the board failure by polling companies last May led to the British Polling Council and Market Research Society setting up an official inquiry.

The work, chaired by Patrick Sturgis, a research methodology professor at Southampton University, produced a main finding of sampling errors indicating pollsters spoke to too many Labour supporters and not enough Tories.

Professor Sturgis said: 'There have been many theories and speculations about what went wrong in 2015 but, having considered the available evidence, the inquiry panel has concluded that the ways in which polling samples are constructed was the primary cause of the polling miss.'

Other factors which could have produced errors were voter turnout misreporting, question wording and ordering, treatment of overseas voters, treatment of postal voters, and treatment of un-registered voters.

The report said there was limited evidence of a 'late swing' to the Conservatives, with some polls indicating this but not others.

The researchers concluded this could only have been a small factor in the error.

A final report is due to be published in March which will make recommendations on correcting the errors at future elections.

Sampling errors were also identified as a key factor by polling expert professor John Curtice in a separate review.

Last week, his report said: 'In short, those who were most easily interviewed by British Social Attitude (BSA) interviewers appear to have been more likely to support Labour and less likely to support the Conservatives to a degree that cannot be accounted for the by the social profile of these respondents.

'If indeed this group in any way mimics the kind of person who was most likely to respond to the polls, then we can begin to understand why the polls might have overestimated Labour's strength.'

Prof Curtice said: 'Opinion polls are intended to provide their journalistic clients with a relatively inexpensive way of securing a reading of the very latest political weather.

Mr and Mrs Cameron, pictured returning to Downing Street following the unexpected Tory win, were able to enter No 10 at the head of the first majority Tory government since 1996

As Mr Cameron, pictured left speaking to the media in Downing Street following his shock win, set to work building his new team, Ed Miliband, right at his resignation speech with his wife Justine, was consigned to defeat despite polling indicating it had been a close race

'They need to be inexpensive because of limited budgets, while a timely reading is often wanted because of a wish to establish whether a recent political event or development has changed public opinion - even though few such events or developments prove to have any electoral consequence.

'As a result polls are conducted in a way that does not fully meet the requirements of random sampling. A key lesson of the 2015 election is that, as a result, they run the risk of failing to take the political temperature correctly.

'Their approach at that election resulted in deficiencies in their samples that subsequent weighting and filtering of the data failed to correct.

'Whereas the polls still largely put Labour and the Conservatives neck and neck even when they asked people after the election how they had voted, two major post-election surveys that used random sampling, BSA and the British Election Study, have both been able to replicate the Conservatives' 6.6 point lead reasonably accurately.'

Prof Curtice, senior research fellow at NatCen, added: 'A key lesson of the difficulties faced by the polls in the 2015 general election is that surveys not only need to ask the right questions but also the right people. The polls evidently came up short in that respect in 2015.

'BSA's relative success in replicating the election result has underlined how random sampling, time-consuming and expensive though it may be, is more likely to produce a sample of people who are representative of Britain as a whole.

'Using that approach is crucial for any survey, such as BSA, that aims to provide an accurate picture of what the public thinks about the key social and political issues facing Britain and thus ensure we have a proper understanding of the climate of public opinion.'

The 2015 British Social Attitudes survey consisted of 4,238 interviews carried out between July 4 and November 2, 2015.

Jeremy Corbyn has the WORST polling record of any Labour leader since World War II

Analysis of the average Labour deficit in opinion polls suggest Mr Corbyn is faring badly compared to his predecessors

Eight months after the last general election, Labour leader Jeremy Corbyn is further behind the Tories than any of its predecessors who tried to recover from election defeat.

The numbers are so bad for the Labour leader it proved impossible to identify a worse performing Labour leader in the history of routine polling - which began in the late 1940s - this far from the last general election.

Eight months into the first majority Tory government since 1996, an average of polls suggests Labour is eight points behind David Cameron's Conservatives.

Pollsters have suggested even this rating could be optimistic given the huge errors made in surveys ahead of the general election.

Some corrections to opinion poll methods have been made since every firm failed to spot the eventual Tory majority but there are fears polls still overstate Labour support.

According to research published today, at this stage in the last parliament, Labour was ahead of the Tories by an average of five points.

And eight months after losing the 1992 election, Labour had opened up a lead of 10 points.

The last time the party was still polling behind the Conservatives this long after an election defeat was in 1988, when it trailed by five points.

But it is impossible to find any record since the Second World War of a gap bigger than eight points at this stage in any electoral cycle.