A growing number of people are planning to boycott this year's census amid increasing fears about data security and the involvement of arms manufacturer Lockheed Martin.

Many campaigners are angry that the £150m contract to run the census has been awarded to the American arms company while others claim the legal safeguards in place to prevent breaches in data security are "so flimsy as to be useless".

Chris Browne, from the Count Me Out campaign, said: "The more people who find out about the involvement of the world's largest arms producer in our census, the more civil dissent we will witness, and the bigger the campaign will get."

Protesters held a day of e-action today in which people were urged to swamp Lockheed Martin a "Twitter storm" and email messages detailing the minutiae of their day. Hundreds more are expected to take part in protests across the country on Saturday, with many saying they are willing to risk a criminal record and a £1,000 fine by refusing to fill in the 32-page questionnaire.

"I have no objection to the census itself because I recognise that it has served an important purpose historically," said Emma Draper, an anti-arms trade campaigner from London:. "However, I think it is outrageous that the government can get away with paying a huge arms company millions of pounds in order to process data which is supposed to be of benefit to public services and people's welfare."

Lockheed Martin, which makes Trident nuclear missiles and F-16 fighter jets, won the £150m contract in 2008. A spokesman for the Office of National Statistics defended the company's involvement, stating it was "a major supplier of non-defence-related services for the public sector".

An ONS spokesman said: "The contract for census processing was awarded to Lockheed Martin UK – not Lockheed Martin US – in August 2008. Lockheed Martin UK offered best value for money in an open procurement under European law and the EU procurement directives were satisfied."

Symon Hill, writer and associate director of Christian thinktank Ekklesia, said many people remained unhappy about the decision and that he would be among those not be filling out the census. "Lockheed Martin is a company that has armed dictatorships around the world that has played a heavy role in the disastrous wars in Afghanistan and Iraq... I have reflected at great length. I have not taken this decision lightly but I feel that being asked to fill in the census is being asked to co-operate with an arms company and, as a Christian and as a pacifist, that is something that I feel I can not do in conscience."

Data from the census forms, which need to be completed by 27 March, is sent to a secure plant in Manchester and then to Titchfeld, Hampshire, for analysis. It is owned by ONS and a spokesman said it remains confidential for 100 years.

The government uses the information to award billions of pounds in grants and a boycott would cause widespread policy and funding difficulties. Councils would be hit hardest, potentially missing out on hundreds of millions of pounds if large numbers of people are not counted.

A spokesman for the ONS said: "The census ... is a unique snapshot of the population on a single day and is vital for your local community."

Lockheed Martin does more than 60% of its work for the US defence department and assists more than two dozen American government agencies. It is also reportedly involved in surveillance and data processing for the CIA and the FBI.

Campaigners fear that because it is a US company and therefore subject to the Patriot Act, which allows the US government access to any data in its possession, US authorities could have access to personal data on the UK's entire population.

The ONS has dismissed this claim, stating: "Under the contractual and operational arrangements we have put in place, no employees of Lockheed Martin UK or of its US parent or of any other US company will be able to access personal census data. The US Patriot Act could not therefore, be used to access such data." The spokesman added that a recent independent review had declared: "the public can be assured that the information they provide to the 2011 census will be well protected and securely managed".

Douwe Korff, professor of international law at London Metropolitan University, has questioned the security arrangements, warning that the "legal safeguards against breaches of confidentiality are so flimsy as to be useless".

"In a democracy under the rule of law, one should not have to rely on blind trust in the authorities; the law should guarantee restraint," he wrote in a recent paper. "However, the law that applies to the census data shortly to be collected does nothing of the sort. It does not stand in the way of the UK police, or intelligence services, or indeed foreign law enforcement agencies and secret services, seeking access – not just in exceptional cases but for general 'trawling' or 'fishing'."

Humanist protest

In a separate challenge to the 2011 census, the British Humanist Association says the wording of the only optional question – "What is your religion?" – is biased and will give "a wholly misleading picture of religiosity in the UK".

Naomi Phillips of the BHA said: "Every other social survey, including the British Social Attitudes survey, asks non-leading questions on religion and has found the number of non-religious people to be at around 30-50%.

"But this wording is hugely biased, assuming people have a religion. In the 2001 census the same wording was used and it found more than 70% of people identified as Christian. This cut the number of non-religious people, according to other surveys, in half."

She said the question was placed in the section on culture and ethnicity in the census and so encouraged people to "think of themselves as 'white, British, Christian' or whatever, which merges ideas of culture, ethnicity and race".

The BHA has run an advertising campaign urging people to tick the "no religion" box on the census, initially with the slogan: "If you are not religious for god's sake say so."

The original posters were refused on buses and at railway stations after the Committee of Advertising Practice advised they had the potential to cause "widespread and serious" offence, and the slogan was changed, on all but two billboards, to: "Not religious? In this year's census say so."

The campaign has 6,500 followers on Facebook and more than 3,000 pledges of support on its website.