The UK will get two specialist ships quicker than planned in the face of Vladimir Putin's "illegal and unprovoked invasion of Ukraine".
The multirole ocean surveillance (MROS) ships will be operated by the Royal Fleet Auxiliary and have been described by the Ministry of Defence as "vital" to national security.
The vessels will be adaptable and able to operate autonomous systems for underwater surveillance and sea bed warfare, with the first due to arrive in January, months ahead of schedule.
Ben Wallace, the defence secretary, said: "In the face of Russia's illegal and unprovoked invasion of Ukraine and Putin's reckless disregard of international arrangements designed to keep world order, it is right that we prioritise delivering capabilities which safeguard our national infrastructure."
He added that further announcements on Naval investments will be made in the coming weeks.
The seabed is increasingly being used for communications and energy supply, placing a greater risk on the West's national infrastructure.
The hi-tech ships will aim to protect the UK's subsea cables and pipelines from enemy attacks.
As a result of the sped-up purchases, the plan to build a new royal yacht, which was going to be named after the Duke of Edinburgh, has been axed, Mr Wallace said.
The flagship vessel, which was due to cost about £200m, was going to be used to host trade fairs, ministerial summits and diplomatic talks.