At least £4.7 million of the controversial fiscal fines dating back to 2010 have yet to be paid, according to new figures.
The Conservatives said the scale of the problem showed the system was at risk of being "reduced to a laughing stock among criminals".
Prosecutors have been issuing the fines since 2008 despite continuing questions over their effectiveness as many offenders simply ignore them and refuse to pay.
Fiscal fines are supposed to be issued for low-level crimes to avoid the need for the accused to appear in court and receive a criminal conviction. However, they have also been used in domestic abuse and serious assault cases.
Scottish Court Service (SCS) figures show that 35.9 per cent of these fines remain outstanding, more than any other type of fine for the three years between 2010/11 and 2012/13.
They also reveal that £2.5m, representing more than one-third of police antisocial behaviour fines over those years, remain unpaid.
Scottish Conservatives justice spokeswoman Margaret Mitchell MSP said: "The fact these so-called lower level fines are even less likely to be settled is extremely worrying.
"Anyone who breaks the law should be left in no doubt that they have to pay these penalties, regardless of where they were sanctioned. If the Scottish Government does not get a grip on this, it risks reducing the justice system to a laughing stock among criminals."
A total of £18.2m remains outstanding across all types of fines, equating to 23 per cent for the three years between 2010/11 and 2012/13. The SCS states £4.8m of that is "on track" to be paid.
According to data released under a freedom of information request by the Conservatives, a further £4.8m remains unpaid prior to this period.
Ms Mitchell said it was the government's duty to ensure the fines were paid. She said: "The public will be appalled to see more than £23m is missing from the Scottish Courts Service because criminals aren't settling fines."
Labour's justice spokesman Graeme Pearson MSP added: "Yet another year goes by with a vast amount of money being left uncollected by the SCS.
"How many more years will go by with millions of pounds of fines outstanding? Fines left unpaid represent those who committed offences ignoring the requirements of justice."
The SCS has collected £61.2m in fines between 2010/11 to 2012/13.
It warns defaulters there are a range of options available for the recovery of fines, including deducting money directly from wages, savings or benefits. In one case, a man from Dumfries was stopped at Manchester Airport for not paying a £200 fine for failing to have proper car insurance. He was only allowed to catch his flight once the penalty was paid.
An SCS spokeswoman said: "Consistently strong collection rates are the clearest indicator that our fines enforcement and sanctions are working well. Naturally, it takes time to collect fines as they can be paid by instalments over the course of two or more years and it is only when defaulters fail to make payment that enforcement measures are used. But that certainly does not mean defaulters can escape payment."
A Scottish Government spokeswoman added: "Since the SCS took over responsibility in 2008, overall fines collection rates have improved considerably and are now at consistently high levels. Their robust approach to pursuing all outstanding fines gives a clear message to defaulters that there is no place to hide.
"Recent figures show that 86 per cent of the value of sheriff court fines imposed between April 1, 2010 and March 31, 2013 were paid or are on track to be paid, and payments directly deducted from benefits reached more than £1.6m in the 10 months from April 2013 to January 2014."