Parents taking their children out of school in Calderdale without authorisation have been fined more than £400,000 since 2015 - but nearly half of that money remains unpaid.
There have been 3,498 penalty notices issued to parents for the unauthorised absence of their children from school since 2015/16, a figure which more than trebled from 500 that year to 1,581 in 2017/18.
Since the start of September last year, 596 fines have been handed out.
Each fine is for £120, but reduced to £60 if paid within 21 days, an amount fixed by the legislation governing the issue of penalty notices.
Of the 3,498 notices issued, according to figures released under the Freedom of Information Act, 1,462 remain unpaid, meaning the council is owed £175,400 from parents.
In a statement, the Calderdale branch of the National Education Union said: "We are very skeptical about the fining system as it penalises the poorest and least advantaged parents, who are the least able to pay.
"Fining the parents also fails to tackle the causes of unauthorised absences which are often linked to wider vulnerability and disadvantage in our community. This is not helped by fining the parents, in fact it is made worse."
Calderdale Council’s Cabinet Member for Children and Young People’s Services, Coun Megan Swift, said: “In partnership with schools, we’re committed to raising attendance amongst all pupils and take instances of unauthorised leave during term time very seriously.
“Penalty Notices are issued in line with national pupil registration regulations and will be considered for two separate reasons; a pattern of regular absence over a fixed period or for an unauthorised leave of absence of over five consecutive days.
“There is no obvious reason for the number of notices issued increasing in recent years, but an increasing number of schools are now requesting that the Council issue Penalty Notices in relation to periods of unauthorised leave of absence during term time.
“The amount of fines can vary depending on whether parents/carers choose to pay the Penalty Notice within three weeks at £60 or after this period, when the cost doubles.
“We always urge people to pay as soon as possible, but where fines are unpaid we will pursue the matter through the courts process.
“Mechanisms have been strengthened to ensure that each matter is dealt with and outstanding fines are paid as soon as possible.
“We are now using the single justice procedure for the majority of our cases, which means that cases can be decided by a magistrate without the need to go to court.
“This process was introduced in September 2018 and is now fully implemented. Previously we had to wait for court time slots at Bradford Magistrates Court, who were experiencing an increased volume of more serious cases which they had to prioritise.”