Soldiers work on a Challenger 2 main battle tank during an exercise near Bordon, Hampshire, England. AFP
"They are in Ukraine already," spokeswoman Iryna Zolotar told AFP, without providing any other details.
The tanks are expected to be involved in any spring counter-offensive launched by Ukraine.
Ukraine's defence minister said earlier Monday he inspected a "new addition" to the country's forces -- Challenger tanks as well as Germany's Marder infantry fighting vehicles, plus Cougar armoured trucks and Stryker armoured personnel carriers from the United States.
"A year ago, no one would have thought that our partners' support would be so strong," Defence Minister Oleksiy Reznikov said on Facebook.
He praised Britain's Challengers, calling them works of "military art".
In London, Britain's government said Monday that Ukrainian crews who have been training to use UK-donated Challenger 2 tanks are now ready to deploy to the frontline.
"Ukrainian tank crews have completed training on Challenger 2 tanks in the UK and have returned home to continue their fight against Russia's illegal and unprovoked invasion," the ministry of defence said in a statement.
The training began shortly after London announced in January that it would send 14 of the tanks to the front line.
The crews learned how to command, drive and "effectively identify and engage targets", said the UK ministry, which called it a "step change" in capability for Ukraine's armed forces.
"It is truly inspiring to witness the determination of Ukrainian soldiers having completed their training on British Challenger 2 tanks on British soil," said Defence Secretary Ben Wallace.
"They return to their homeland better equipped but to no less danger. We will continue to stand by them and do all we can to support Ukraine for as long as it takes," he added.
Lieutenant Colonel John Stone, who oversaw the training mission, said the British instructors were "hugely impressed with the level of competence displayed" by the Ukrainian crews.
Russian President Vladimir Putin said last week that Moscow would be "forced to react" if Britain follows through on a promise to deliver to Ukraine armour-piercing ammunition containing depleted uranium.