The British army has been accused of targeting its headline-grabbing “snowflake” recruitment campaign at young people when they were facing a post-holiday low.

A briefing document seen by the Guardian shows that strategists behind the “Your army needs you” campaign factored in that it would be seen by young people at a time when they were experiencing the “January blues”.

Campaigners who oppose the British policy of recruiting boys and girls aged 16 and 17 said it was another example of young people being targeted at times – such as exam results day – when they may be most vulnerable.

The high-profile campaign called on “snowflakes, selfie addicts, class clowns, phone zombies, and me, me, me millennials” to sign up. It suggested that what was seen as a weakness or a character flaw by the rest of society could be regarded as a strength by the army.

The briefing document says social media promotion of the campaign was organised to coincide with television adverts shown on one of the bleakest weekends of the year.

It said: “Social should be synced with the biggest TV shows that we are launching in and should account for upweights during the launch weekend, especially on the Saturday and Sunday 5th and 6th of Jan when the ‘January blues’ are setting in.”

The ad appears to have been highly successful. According to the outsourcing giant Capita, which has handled recruitment for the army since 2012, more than 1,000 additional people started the process to join the army after the campaign’s release, and applications reached a five-year high.

Some 16,000 people applied to join in January, and visits that month to the army’s website were up by 78% to 1.5m – twice the traffic for the previous year.

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Charlotte Cooper, UK research and campaign officer at Child Soldiers International, said: “This advertising campaign’s targeting of young audiences feeling the ‘January blues’ is another example of how the army tries to exploit young people’s emotional vulnerability to drive recruitment, instead of encouraging a fully informed, mature and rational decision over a potentially life-changing commitment.

“The snowflake campaign tried to present a new side to the army, but this advertising brief shows the same old story: young people with the fewest options being mis-sold a one-sided view of military life as the magic ticket to a better life.”

Liz Saville Roberts, the Plaid Cymru MP, also expressed concern that the briefing document appears to show the army continuing to focus on cities in northern England and south Wales and on people from “C2DE” backgrounds, marketing speak for the lowest three social and economic groups.

She said: “Once again, we have found the government’s recruitment ads specifically looking for young people from deprived areas, like Cardiff and Swansea, to fill its huge gap in infantry soldiers.

“This is all under the false mask of being the democratiser of opportunity whilst targeting those who it sees as having limited opportunities in the first place.”

The army pointed out that most of its campaigns were released in January and denied that it only targeted those from C2DE families or areas of deprivation.

Col Ben Wilde, assistant director of recruiting, said: “Like all modern national recruitment campaigns, our adverts are designed to reach a broad range of audiences. This enables our armed forces to be representative of the nation they help to keep safe.

“It should be no surprise that a national recruitment campaign would focus on major population centres across the UK, or increase activity in the new year when individuals are looking for new roles.”

Speaking at an advertising industry event on Wednesday, Matthew Waksman, planning director at Karmarama, the agency that created the campaign, explained how the army knew that there would be an initial negative reaction to it and that they and the army were “ready for the backlash”, the Drum reported.

“[The army has] been a Stonewall top employer for ages, it’s got a less than 1% pay gap, it’s been supporting soldiers through transition for a long time, and so the army was ready to talk about what it means to be a progressive organisation,” he said.