Tymoshenko said on Saturday that she will fight the result of the vote in Ukrainian courts, calling Yanukovych's win illegitimate.
"I want to clearly state: Yanukovych is not our president. Whatever happens in future, he will never become the legitimately elected president of Ukraine," she said in a televised broadcast.
Tymoshenko's team has claimed widespread violations took place during voting, particularly in Yanukovych's eastern Ukraine strongholds, but international observers hailed the vote as a fair and clean.
"There are still complaints that have not been reviewed by the courts. The result has been announced too early," Volodymyr Pilipenko, a Tymoshenko delegate, said after the results had been confirmed on Sunday.
It now seems likely that Tymoshenko's supporters will present their alleged evidence of election fraud to a Kiev court.
There are concerns that the prime minister's continuing refusal to concede the presidential race will lead to political deadlock in the country.
Yanukovych has asked Tymoshenko to step down as prime minister but she has so far refused.
If she is voted out by a vote of no-confidence, which political commentators consider likely, she will stay on as acting prime minister until a new parliamentary coalition is formed - a long and complicated process.
If no coalition can be put together, the president-elect might have to dissolve parliament and call new parliamentary elections
The stalemate also threatens the prospects for economic recovery and the resumption of International Monetary Fund (IMF) lending.
The IMF suspended a $16.4bn bailout programme late last year because of breached promises of fiscal restraint.