WARSAW, April 23 (Reuters) - Prime Minister Rishi Sunak said on Tuesday he would lift defense spending to 2.5% of GDP to reach 87 billion pounds ($108 billion) a year by 2030, saying Britain could not be complacent when the world was at its most dangerous since the Cold War.

Sunak has been under pressure from his governing Conservative Party to boost defense spending more quickly after he argued he could do so only "as soon as economic conditions allow."

The rise, from around 2.32% of gross domestic product, will not only be cheered by his party before this year's election that the Conservatives are expected to lose, but could weaken potential leadership challengers who have championed defense.

It will also be welcomed by Jens Stoltenberg, secretary-general of the NATO military alliance, who has been calling on other members to meet their 2% defense spending commitments.

"In a world that is the most dangerous it has been since the end of the Cold War, we cannot be complacent," Sunak said in a statement before a news conference with Stoltenberg. "As our adversaries align, we must do more to defend our country, our interests, and our values."

"Today is a turning point for European security and a landmark moment in the defense of the United Kingdom. It is a generational investment in British security and British prosperity, which makes us safer at home and stronger abroad."

The government said defense would get an additional 75 billion pounds over six years, making Britain the second-largest defense spender in NATO.

The announcement followed Sunak's pledge to increase Britain's military support for Ukraine by 500 million pounds to take its total for this financial year 3 billion pounds - a move welcomed by Ukrainian President Volodymyr Zelenskiy.

AIR DEFENCE

Earlier, talking to reporters traveling with him, Sunak said he was slightly late for his flight to the Polish capital Warsaw because he had spoken to Zelenskiy about the new package and Ukraine's need for more air defense systems.

Zelenskiy has repeatedly called for more air defense systems to protect Ukraine's people from increasing Russian bombardment, and Germany has spearheaded calls for members of the NATO military alliance and beyond to step up on deliveries.

Asked whether Britain was backing Germany's proposal to find more air defense systems, Sunak reiterated what Britain had already delivered, and said the new package - including 60 boats, more than 1,600 strike and air defense missiles and nearly 4 million rounds of ammunition - would also help Ukraine.

Britain has deployed the anti-air and anti-missile Sky Sabre system to Poland to help its defenses.

"So I agree that air defense is one of the critical areas where Ukraine needs support. And indeed, actually I was on the phone with Zelenskiy this morning," Sunak told reporters.

"And he and I were talking about air defenses, one of the many things that we were talking about," he said, adding he would discuss the issue with Stoltenberg, Polish Prime Minister Donald Tusk and, on Wednesday, German Chancellor Olaf Scholz.

Zelenskiy welcomed the new package, saying on X: "Storm Shadow and other missiles, hundreds of armored vehicles and watercraft, ammunition — all of this is needed on the battlefield."

Sunak repeated his argument that Britain had frequently been first to deliver weapons to Ukraine and called on other European countries to fulfill their NATO commitments.

"It's also right we do our bit and that's why it's important Europeans invest in their own security at the same time. That's what we've always done," Sunak said. "It was very welcome news over the weekend from the U.S. but that doesn't take away from the need for Europeans to invest in their security."

Sunak's visit to Poland and Germany is his first international trip for months and is aimed at showing his restive party he is still in command ahead of an election later this year.

($1 = 0.8102 pounds) (Reporting by Elizabeth Piper Editing by Ros Russell and Bernadette Baum)