The UK has been helping Ukraine protect its networks from cyber attacks since the start of a full-scale Russian invasion on 24 February.
This is reported by Sky News.
Leo Docherty, the deputy foreign secretary, said the damage would have been "very significant" without British help, adding that Britain had been forced to strengthen its own defenses because of the "very significant cyber threat from Russia".
"We are already on the front line... We are aware of the threat. We have strengthened our own training and our own protection," he said.
"That is why we have deep experience in the field of cyber defense and that is why we were very happy to share it with our Ukrainian allies," the official added.
Normally such activities, which have so far cost the UK around £6.35 million, are kept secret, but it is believed that Russia is aware of the UK's involvement, so the need for secrecy has disappeared.
According to Dockerty, the intrusion was accompanied by a significant increase in cyber attacks. The involvement of UK cyber security experts and experts from other countries has been able to limit the impact of cyber attacks.
Building on the experience of the National Cyber Security Center (NCSC), the UK is playing a leading role in cyber defence.
Dockerty noted that the support is similar to the indirect support given to Ukraine through the weapons program and therefore cannot be seen by Russia as direct intervention in the war.
"This is an absolutely honorable and non-escalating act. Ukrainians are on the front lines of the defense of their homeland and ... we have provided some material assistance and expertise to allow them to do this very, very effectively," he said.
Foreign Secretary James Cleverley added: "The UK's support for Ukraine is not limited to military aid - we are using the UK's world-leading experience to support Ukraine's cyber defences. Together we will ensure victory over the Kremlin in all areas: on the ground, in the air and in cyberspace."
The support provided includes hardware and software that can prevent attacks while ensuring people have access to vital information and services. Forensic support is also provided to help Ukraine determine where and how the attacks were carried out.