The advertising watchdog has decided not to launch a formal investigation into a controversial advertisement from the Christian party proclaiming that "there is definitely a God", even though it has become one of the four most criticised adverts of all time.

The advertisement was unveiled by the party last month in response to the British Humanist Association's bus adverts, which state: "There's probably no God. Now stop worrying and enjoy your life." The Christian party's advert – displayed on 50 London buses – carries the slogan: "There definitely is a God. So join the Christian party and enjoy your life."

Figures from the Advertising Standards Authority reveal that the advertisement has so far attracted 1,045 complaints – and rising – making it the fourth most complained about advert since the ASA's records began. But it has decided not to launch an investigation because the poster is deemed to be 'electioneering material', and falls outside the remit of its codes of practice.

The ASA says it has been deluged with complaints since the beginning of the year, indicating the extent to which advertising has become the latest battleground between conflicting ideologies. Last month the Christian party's London headquarters were vandalised in an attack which police believe was a religious hate crime.

A spokesman for the ASA said today: "We will not be investigating the Christian party advert. We do not play the numbers game, it is about measuring the complaints against the advertising codes and judging whether or not there appear to be a problem under the rules." He said the Christian party ad was not investigated because the primary purpose of the poster was to promote the party.

"The advert was electioneering material as defined by clause 12.1 and therefore not subject to the code." The clauses states: "Any advertisement or direct marketing communication, whenever published or distributed, whose principal function is to influence voters in local, regional, national or international elections or referendums is exempt from the Code."

In January the ASA concluded that the aetheist "There's probably no God" bus ad campaign by the British Humanist Association did not breach the current advertising code and again decided not to launch an investigation.

That advert attracted 326 complaints, some complaining that the ad was offensive and denigratory to people of faith. Others challenged whether the advert was misleading because the advertiser would not be able to substantiate its claim that God "probably" does not exist. The ASA council concluded that the ad was an expression of the advertiser's opinion and that the claims in it were not capable of objective substantiation. Although it acknowledged that the content of the ad would be at odds with the beliefs of many, it concluded that it was unlikely to mislead or to cause serious or widespread offence.

People complaining about the Christian party advert believe the claim "there definitely is a God" is misleading because it cannot be substantiated, while some individuals have also objected that the advert is offensive to atheists. But the ASA spokesman said that even if the advert had fallen within the remit of its code, it was unlikely it would have investigated because of its decision not to investigate the aetheist bus campaign. The codes are being reviewed to streamline them and a public consultation is expected to be launched before the end of the month.

The ASA has also decided not to investigate two other advertising campaigns of a similar nature. An advertisement from the Russian Orthodox Church that stated "There IS a God, BELIEVE. Don't worry and enjoy your life" was, the ASA council considered, a reflection of the opinion of the advertisers and unlikely to mislead readers.

Similarly, the Trinitarian Bible Society's ad that claimed "The fool hath said in his heart, There is no God. Psalm 53.1 " generated complaints that it was offensive and was insulting to atheists and non-Christians. Some also objected that the ad was misleading because it implied that God exists.