The National Crime Agency (NCA) said 10 known sex offenders travelled to the country in the six weeks following the Russian invasion.
All 10 men were asked to leave Poland following an interview with immigration officers and law enforcement, and British authorities are working to deter others from travelling.
“For us, it’s about making partners aware that these convicted child sex offenders are on their turf, for obvious reasons,” an NCA spokesperson said.
“There are at least 5,000 unaccompanied children who have been displaced from Ukraine. Making sure they’re safe is absolutely paramount.”
Officials said convicted paedophiles are meant to declare their criminal record to countries they are visiting but “we find inevitably they haven’t”.
Efforts are currently centred around preventing sex offenders from travelling to Poland or other countries if not “for the right reasons”, and getting them out if they have not followed the rules.
The issue is among a “whole range of threats that have sprung up or been enhanced by this conflict” Ukrainian war, the NCA said.
They include modern slavery, human trafficking and organised immigration crime affecting refugees.
Police and security services are also concerned about the potential “leakage” of firearms and weaponry.
There have also been increases in illicit financing, cybercrime and fraud, including the setting up of websites claiming to be aid agencies in a bid to take donations meant for Ukrainians.
The NCA has been leading the UK’s response to organised crime influenced by the Ukrainian war and has also created a Combating Kleptocracy Cell to target “corrupt elites” linked to Russia and other hostile states.
A kleptocracy describes the corruption that occurs when state leaders and those linked to them take money generated from within their nations, such as from natural resources, for personal gain and spend or stash it in countries elsewhere.
The cell is focused on individuals who have used assets in the UK to store, hide and grow their wealth.
The cell has been using covert tactics to capture sensitive intelligence to try and inform decisions by the government and private sector.
It is also targeting “enablers”, which can include auction houses, estate agents, lawyers and security companies. Several arrests have been made but no charges have yet resulted from the activity.
The Combating Kleptocracy Cell does not specifically target Russia, but the country is one of several said to have “corrupt elites” that British authorities now want to disrupt.
The NCA believes its efforts are already having a deterrent effect, with some targets linked to Vladimir Putin’s regime already divesting themselves of assets in the UK, as Russian-owned property in the United Arab Emirates and other nations explodes.
The NCA said it wanted to push back on the “concept of Londongrad, the idea that people were free to come with their money and do whatever they liked”.
“There’s always going to be opportunities for people to move their assets somewhere else,” a spokesperson added. “Clearly some people have decided the UK is no longer for them.”