Ukraine's presidential election is a "decisive signal of their support for reform", William Hague declared as he praised voters for defying disruption by pro-Russian separatists.
The Foreign Secretary said every vote cast was "an individual act of courage" as he pledged to work with the country's new leader on ambitious reforms.
Exit polls suggested a convincing win for billionaire chocolate tycoon Petro Poroshenko, who is said to have received 55.9% - with former prime minister Yulia Tymoshenko a distant second.
Voting was severely disrupted in parts of the east of the country, where separatists have seized control amid fighting that has claimed at least 20 lives in recent days.
Kiev and the West accuse Moscow of backing the insurgents following its annexation of the Crimea region, leading to the imposition of sanctions by the EU and US.
"I congratulate the Ukrainian people for showing their determination to vote in today's historic presidential elections," Mr Hague said last night.
"Today's high turn-out shows the Ukrainian people's resolve to decide their own future without outside interference.
"I regret that separatist armed gangs who purport to be 'peaceful activists' sought to deny the citizens of Donetsk and Luhansk their right to vote.
"I pay tribute to the election commission staff who were subjected to appalling levels of intimidation in those regions but strove to do their duty.
"I also praise citizens in eastern Ukraine who overcame all obstacles to vote or who tried to do so. Each vote cast was an individual act of courage.
"The UK, like the vast majority of the international community, is firmly committed to the sovereignty, independence and territorial integrity of Ukraine.
"That is why we will not recognise Russia's illegal annexation of Crimea.
"Today, Ukrainians have sent a decisive signal of their support for reform and a new future for their country.
"We look forward to working with the new president on ambitious reforms that can meet the aspirations of the people of Ukraine."