A "new Iron Curtain" threatens to fall across Ukraine, ex-Prime Minister Baroness Thatcher has warned.
Lady Thatcher said the West must act decisively
She issued a statement after Foreign Secretary Jack Straw said the UK could not accept the country's presidential elections were either free or fair.
Ukraine's supreme court is considering claims of fraud in the poll but outgoing President Leonid Kuchma has suggested re-running the election.
Urging the West to act, Lady Thatcher said: "Tyranny must not prevail."
Lady Thatcher was prime minister when Mikhail Gorbachev began the reform programme which came before the break-up of the Soviet Union.
Supporters of Prime Minister Viktor Yanukovych have said they will hold a referendum on autonomy if opposition leader Viktor Yushchenko is declared winner by the supreme court.
That has raised the prospect of the country being split.
Mr Straw appealed for calm in Ukraine during a wide ranging statement in the House of Commons adding that the situation in the country remained fragile.
He told MPs the UK's ambassador had delivered a message to the country's interior minister urging all sides to show restraint.
Tory Michael Ancram said the only way out was to rerun the 21 November vote.
EU foreign policy chief Javier Solana, has said "the unity of Ukraine is fundamental".
Speaking in Brussels after a meeting with EU and Nato ambassadors, he said dialogue must continue in Ukraine and "what we have to avoid is the outbreak of violence".