Foreign Secretary Liz Truss said the pair's "reckless behaviour" in attempting to bring about the de facto secession of Republika Srpska -- one of Bosnia and Herzegovina's two entities -- had been "encouraged" by Russian President Vladimir Putin's example.

One half of the country belongs to the Serb entity Republika Srpska, while the other is ruled by a Muslim-Croat federation.

"These two politicians are deliberately undermining the hard-won peace in Bosnia and Herzegovina. Encouraged by Putin, their reckless behaviour threatens stability and security across the Western Balkans," she said.

"With these tough sanctions we are showing that the enemies of peace will be held to account."

The pair now face British travel bans and asset freezes and are the first to be targeted under London's Bosnia and Herzegovina sanctions regime.

Fears of destabilisation have mounted in Bosnia in recent months since Bosnian Serb leader Dodik, a Kremlin ally, issued secessionist threats.

The UK said that both individuals "have used their positions of authority to push for de facto secession of Republika Srpska... in direct contravention of the country's constitution."

In December, Dodik launched a process of Serb withdrawal from Bosnia's joint army, judiciary and the tax system, stirring fears of breaking up the country or starting a new conflict.

Before making this move, Dodik travelled to Moscow, appearing to have tacit backing from Russia, the Serbs' historic ally.

Republika Srpska President Zeljka Cvijanovic in February spoke about Serbs being "frustrated over constant efforts aimed at destroying their identity," amid threats to secede in a bid to rid Bosnia's top court of foreign judges.

Three foreign judges serve on the court along with two Croats, two Muslims and two Serbs -- a legacy of the peace deal that ended the 1992-1995 war and divided Bosnia along ethnic lines.

Dodik, who is the Serb member of Bosnia's joint presidency, termed the British sanctions "nonsense".

"I don't give a damn," he told Bosnian Serb lawmakers, adding: "Great. They never had good intentions towards Serbs".

Dodik said he had no property in Britain nor had he visited the country in the past 10 years.

The top international envoy to Bosnia, Christian Schmidt, said the British action was "reasonable".

The two politicians "will have to bear the consequences of their words and deeds", he said in a statement.

Washington in January imposed fresh sanctions on Dodik, whom they had already sanctioned in 2017.

The US accused him of threatening the stability of Bosnia and the whole region with his "corrupt activities" and attempts to dismantle the Dayton peace deal.

In recent weeks Dodik has strongly criticised Bosnia's ambassador to the UN for backing resolutions against Russia's invasion of Ukraine, and argued that the position of Sarajevo was not decided by the joint presidency.