Tymoshenko, the fiery 49-year-old prime minister who trailed Yanukovich by only a slim margin in the 7 February vote, renewed charges of election fraud against his camp on Saturday and said he would never be a "legitimately-elected" president.
But supporters of the 59-year-old opposition leader, who is backed by wealthy industrialists, brushed off any real threat from an expected challenge by Tymoshenko to a high court.
One pro-Yanukovich official said that after the Central Election Commission's declaration on Sunday Yanukovich could be sworn into office as early as the last week of February.
"The Central Election Commission declares Viktor Fedorovich Yanukovich elected president of Ukraine," Commission President Volodymyr Shapoval said after confirming results that showed Yanukovich had defeated Tymoshenko by 3.48 percentage points.
It now seems likely that her supporters will soon present to a Kiev high court evidence of election fraud which Tymoshenko says has been amassed. Analysts say the court could take several days to consider the appeal.
Some of her representatives on the commission said they may appeal against the way the body had conducted its business on Sunday. But many analysts say that Tymoshenko lacks sufficient proof to persuade a court to overturn the commission's findings.
No mass protests
Tymoshenko, a charismatic figure with a populist touch, has promised not to call people out in mass protests like those of the Orange Revolution, when she helped lead a successful challenge to Yanukovich's election victory in 2004 and had it overturned as fraudulent.
But she argues that cheating by the Yanukovich camp cost her more than one million votes that would have tipped the balance in her favour.
The official margin between the two was about 880,000 votes.
This time she is also running against the wishes of the West where governments have put pressure on her to recognise, in the national interest, that the fight is over.
International monitors hailed the vote as an "impressive display" of democracy.
US President Barack Obama and other Western leaders, who regard the election as a democratic event rarely seen in former Soviet space, have congratulated Yanukovich.
Yanukovich, an ex-mechanic from the Donbass mining region, is expected to tilt Ukraine back towards Russia, its former imperial master, after five years of estrangement under pro-Western President Viktor Yushchenko.
Yanukovich, in an interview aired by Russian television, said Ukraine may allow Russia to station its Black Sea Fleet in the port of Sevastopol beyond a scheduled withdrawal in 2017.
He also said he was keen to improve gas relations with Russia and would revive the idea of a gas consortium that would allow Moscow to co-manage Ukrainian pipelines.
Prolonged uncertainty
The Tymoshenko camp says Western leaders have backed his election because they are fearful of unrest breaking out in the ex-Soviet state of 46 million. Her continued refusal to concede victory to Yanukovich or heed his call to resign as prime minister spells continuing turmoil for Ukraine.
Analysts say this threatens prospects for a quick economic recovery and early resumption of much-needed International Monetary Fund lending to the country.
The IMF suspended a $16.4 billion bail-out programme late last year because of breached promises of fiscal restraint.
There is no early end in sight to political infighting that has hindered foreign investment in Ukraine.
Yanukovich has asked Tymoshenko to step down as prime minister but she is unlikely to quit voluntarily even after Yanukovich's inauguration, political commentators say.
If she is voted out by a vote of no-confidence, which is likely, she will stay on as acting prime minister until a new parliamentary coalition is formed - a long and complicated process of horse-trading.
If no coalition can be put together, Yanukovich may reluctantly have to dissolve parliament and call new parliamentary elections - only prolonging uncertainty in the country.
(EurActiv with Reuters.)
