The British government said on Tuesday that it would purchase a dozen F-35A fighter jets capable of firing tactical nuclear weapons in what it described as the biggest expansion of its nuclear deterrent in a generation.

The purchase of the Lockheed Martin jets would allow Britain's air force to carry nuclear weapons for the first time since the end of the Cold War, Downing Street said.

"In an era of radical uncertainty, we can no longer take peace for granted, which is why my government is investing in our national security," Prime Minister Keir Starmer said in a statement.

Britain is increasing defence spending and upgrading its military forces, including its submarine fleet, as it faces increasing hostility from Russia and as the United States retrenches from its traditional role as a defender of European security.

The British government said the purchase of the jets would allow it to contribute so-called dual-capable aircraft to NATO to carry nuclear weapons in the event of a conflict.

NATO Secretary General Mark Rutte said "this is yet another robust British contribution to NATO".

Britain's nuclear deterrent currently rests solely on the Trident submarine-based system, which misfired during a test last year, the second successive test failure after one veered off course in 2016.

The last time Britain possessed an independent air-launched nuclear capability was in 1998 when the WE-177 free-fall bomb was withdrawn from service, according to Britain's parliament.

Tactical nuclear weapons are intended for battlefield use, as opposed to strategic weapons designed to be fired across vast distances.

By purchasing the F-35A fighter jets, Britain would be able to diversify its military options and align more closely with NATO allies such as France, and the United States, which maintains land, sea, and air-based nuclear capabilities.

The F-35A fighter jets are capable of carrying U.S. B61 tactical nuclear weapons. Britain would likely need the United States to supply those weapons for use on the planes, said one British official who declined to be named.

The United States withdrew its last nuclear weapons from Britain in 2008, in a sign at that time that the threat of conflict following the end of the Cold War was receding.

Downing Street said buying the new jets would support about 20,000 jobs in Britain and underline its commitment to NATO.

The government has pledged to boost overall defence and security spending to 5% of economic output by 2035 to meet a NATO target and said on Tuesday it must "actively prepare" for war at home for the first time in years.