With court delays owing to the pandemic being blamed for changes in the normal flow of inmates, the Howard League for Penal Reform said court backlogs had put additional strain on defendants and victims alike.
Ministry of Justice data shows that there were 338 inmates at North Sea Camp prison as of March 31 – 75 fewer than at the same point in 2020.
This is in line with the situation across England and Wales, where the number of prisoners fell by six per cent to 78,058 at the end of March.
A MoJ report highlighted the number of people held on remand – those in prison awaiting trial or sentencing – as of March 31, reached a ten-year high last month.
It said: "This quarter we have continued to see the effects of the Covid-19 pandemic on the prison population.
"Between March 31 2020 and March 31 2021, the total prison population has fallen by around 4,900, however this overall figure masks the trends seen for certain population groups.
"The remand population has increased by 22 per cent over the past year, to 12,262. This is the highest ‘as at March 31’ remand population figure for ten years.
"Conversely, the sentenced prison population as at March 31 2021 has fallen by ten per cent over the past year, to 64,783, which represents the lowest level for 15 years.
"This is in line with the effects of Covid-19 on the criminal justice system – in particular, delays in court hearings.
"The effect of this on the prison population is that the normal system flow of individuals from the remand to the sentenced population has been disrupted, resulting in more people held on remand, and fewer sentenced prisoners."
Over the last ten years, there has been a steady falling trend in the number of new prisoners entering the facility each year.
However, as a result of disruption to court processes due to the coronavirus pandemic, the decrease has been sharper.
Crime also fell during the lockdowns.
Frances Crook, chief executive of the Howard League for Penal Reform, said: "The backlog in the courts has put additional strain on everyone affected by crime – not only defendants spending longer in prison awaiting trial, but also victims, who are having to wait even longer for their cases to be heard.
"Now, more than ever before, it is vital that police and prosecutors use their discretion and make sensible decisions to ensure that people are not swept into the system unnecessarily."
A spokesman for the MoJ said: "Courts have been prioritising the most urgent cases throughout the pandemic to protect the public and ensure offenders continue to face justice.
“Thanks to measures such as Nightingale courts and the rapid expansion of videos hearings, outstanding magistrates’ cases have fallen by 50,000 since last summer.”