Banja Luka's Nezavisne Novine daily of Saturday quoted Ashdown as saying that another possible measure was a ban on the ruling Serb Democratic Party (SDS) and replacement of its officials.
Ashdown issued the warning at a meeting on Mt. Jahorina near Sarajevo that was called on Thursday by Republika Srpska President and SDS leader Dragan Cavic, and that was also attended by other senior Serb officials.
"The SDS Presidency is now expected to exert pressure on its deputies in the Republika Srpska Parliament to adopt the police reform," a senior SDS official told the daily.
If the police reform is not adopted, Republika Srpska could end up like the isolated part of Cyprus, the official said.
The European Commission insists on the adoption of three EU principles - the transfer of police authority from entity governments to the central government, the organisation of police force into ten districts crossing entity borders, and the depoliticising of police.
The Serb side insisted at talks held last Wednesday on keeping its own interior ministry with five police districts which would not cross entity borders.
The talks are scheduled to continue in Mostar on August 23.