The Storm Shadow cruise missile has a range of over 250km (155 miles), according to the manufacturer.
By contrast, the US-supplied Himars missiles used by Ukraine only have a range of around 80 km (50 miles).
The weapons will give Ukraine the "best chance" of defending itself, UK Defence Secretary Ben Wallace said.
They are fired from aircraft, so the longer range means Ukrainian pilots will be able to stay further from the frontlines.
Once launched, the Storm Shadow drops to low altitude to avoid detection by enemy radar, before latching onto its target with an infra-red seeker.
The announcement was made in the House of Commons by Mr Wallace. The decision follows repeated pleas from Ukraine for more weapons from the West.
Mr Wallace said the missiles would "allow Ukraine to push back Russian forces based on Ukrainian sovereign territory".
He said the UK took the decision after Russia "continued down a dark path" of targeting civilian infrastructure in Ukraine.
Mr Wallace wrote to his Russian counterpart Sergei Shoigu in December, he said, to warn that further attacks could result in the UK donating more capable weapons.
He said the missiles were "going into" or already in Ukrainian hands, and described the move as "calibrated and proportionate to Russia's escalations".
"None of this would have been necessary had Russia not invaded," he said.
He said the missiles would be compatible with Ukraine's existing, Soviet-era planes and praised the technicians and scientists who made that possible.
But he warned the range of the British-supplied Storm Shadows was "not in the same league" as Russia's own missile systems - with some of Moscow's weapons being able to travel far further.
Earlier this year, Ukrainian Defence Minister Oleksii Reznikov insisted longer-range missiles would not be used to attack targets within Russia itself.
"If we could strike at a distance of up to 300 kilometres, the Russian army wouldn't be able to provide defence and will have to lose," he told an EU meeting.
"Ukraine is ready to provide any guarantees that your weapons will not be involved in attacks on the Russian territory."
In February, UK Prime Minister Rishi Sunak said he was prepared to send long-range missiles to Ukraine, and the British government opened a bidding process for their procurement.
"Together we must help Ukraine to shield its cities from Russian bombs and Iranian drones," Mr Sunak said then. "That's why the United Kingdom will be the first country to give Ukraine longer-range weapons."
On Thursday, Kremlin spokesman Dmitry Peskov said Moscow would take an "appropriate" military response to any British-supplied Storm Shadow weapons used by Ukrainian forces.
The Storm Shadow missile has been operated by both British and French air forces and has been used previously in the Gulf, Iraq and Libya.
The British-supplied missiles can only be fired by aircraft, but French missiles can be fired from ships and submarines.