Critical industries and organisations have been briefed on the need to bolster cyber defences because of the risk of an attack linked to the Ukraine crisis.

Although there is not thought to be a specific threat, the head of GCHQ spoke to representatives from key sectors to highlight advice issued about protecting their systems.

Hackers linked to Russia have been blamed for a string of major cyber attacks by US and British intelligence agencies in recent years.

As tensions have increased in eastern Europe, Ukraine’s banks and defence ministry have been targeted by online attacks.

In response to the crisis, GCHQ director Sir Jeremy Fleming spoke to representatives of critical national infrastructure organisations to “press home our advice at this time of uncertainty”, the National Cyber Security Centre (NCSC) said.

Critical infrastructure covers organisations including the emergency services, energy and nuclear power, food supply, transport, communications and defence.

The talks were “not based on any specific intelligence but a general warning”, a source said.