Prime Minister Rishi Sunak will give details of the 500 million pound ($620 million) package during a visit to Poland, London said on Tuesday.
Ukraine has been desperately calling for more aid as Russian forces began making advances on its front lines and stepped up airstrikes.
Britain made the announcement at a time when the White House is trying to push through a long-delayed $60 billion aid package from the United States.
The money will bring the total UK military aid to Ukraine to 3 billion pounds ($3.71 billion) this fiscal year, the British government said in a statement.
The new package will fund ammunition, air defense systems, drones and engineering support. The government said drones "will be procured in the UK and the funds will support a scale-up in local defense supply chains."
What the UK is calling "the largest ever single equipment package" will also include 60 boats and more than 1,600 offensive and defensive missiles, as well as Storm Shadow long-range, precision-guided missiles.
More than 400 vehicles, including 162 armored vehicles, will also be delivered, as well as 4 million rounds of small arms ammunition.
"Defending Ukraine against Russia's brutal ambitions is vital for our security and that of all of Europe. If Putin is allowed to succeed in this war of aggression, he will not stop at the Polish border," Sunak said.
In Warsaw, Sunak will discuss bilateral relations with Polish Prime Minister Donald Tusk and European security and support for Ukraine with NATO Secretary General Jens Stoltenberg. Sunak will then travel to Germany for talks with Chancellor Olaf Scholz in Berlin.
UK Defense Secretary Grant Shapps said the package would help restore stability in Europe.
"The UK was the first to provide NLAW missiles, the first to deliver modern tanks and the first to send long-range missiles," Shapps said.
"Now we are going even further. We will never let the world forget Ukraine's fight for existence, and with our continued support they will win."
The British prime minister's office said Sunak also had a proposal for Poland, offering to "deploy an RAF Typhoon squadron to conduct NATO air surveillance over Poland next year."
Ukraine's President Volodymyr Zelensky is seeking more help from the West. Russia has made relative military gains in recent months and continues to launch attacks on Ukrainian infrastructure.
The US House of Representatives passed a $60 billion military aid package for Ukraine on Saturday. The package awaits approval by the Senate, which is expected to begin negotiations this week.
