BRITAIN is “secretly” preparing for outright war with Russia, it is claimed.

Government officials are reportedly being ordered to update decades old contingency plans amid fears the nation is not ready for a potential attack from the Kremlin.

The classified “home defence plan” would lay out how Downing Street will respond if Vladimir Putin declares war on the UK, including putting the Royal Family into bunkers.

Ministers worry Britain could be outfought by Russia on the battlefield, but also poorly defended at home as things stand.

Experts have warned the country’s national infrastructure is vulnerable ahead of the release of Labour’s Strategic Defence Review – an examination of the Armed Forces.

It comes as the former Soviet nation continues to threaten the UK over its support for Ukraine – and ministers last month officially designated it a national security threat for the first time.

The ongoing conflict, which began with a Russian invasion in February 2022, could soon see British troops deployed on the frontlines.

The classified plan by the Cabinet Office’s Resilience Directorate – which was last updated in 2005 – would set out a strategy for the days immediately after a strike on British shores, including from nuclear warheads.

For the first time, it’s understood the document – unlikely to ever be released to the public – will address cyber warfare.

It will also direct PM Sir Keir Starmer on how to run a wartime government, as well as strategies for travel networks, courts and the postal system, reports the Daily Telegraph.

The Cabinet Office ran a risk assessment in January in which it modelled a scenario in which a hostile foreign state launched various types of attack on the UK’s infrastructure.

It found any such successful strike could be devastating.

Meanwhile, defence officials have warned Britain must develop its own version of Israel’s Iron Dome to protect against missiles.

A senior RAF official said last month if Russia had attacked the UK in the same way as it had Ukraine more than three years ago – within hours missiles would have broken through defences and destroyed key targets.

There is particular concern for the country’s five active nuclear power stations, which if damaged by strikes could release radioactive material across the country, resulting in long-term impact, according to the assessment.

The plan is based on the War Book, a Cold War dossier of instructions for government response to nuclear attacks.

Under such plans, Britain could be divided into 12 zones, each governed by Cabinet ministers, and food rationed.

A Government spokesman said: “The UK has robust plans in place for a range of potential emergencies that have been developed and tested over many years.”

The Sun has approached the Ministry of Defence for comment.