Key Reformist Bloc (RB) member Bulgaria of Citizens Movement (DBG) has declared it will refuse to back a new coalition government if an anti-corruption bill doesn't make it to Parliament's floor.

Dimitar Delchev, a DBG lawmaker, has told reporters at a briefing that a decision of a Parliament committee on Wednesday to drop the bill is a clear sign of a lacking "anti-corruption majority".

The bill should be included in next week's agenda if DBG, one of the two heavyweights in the RB, is to back any attempts at forming a new coalition, Delchev has argued, accusing MPs at the Committee of holding an "irregular" vote.

The Reformist Bloc took the government mandate on Tuesday, amid disputes among its members over whether it should be returned without any effort to forge a new coalition.

The other big RB party, the UDF, maintains it would seek a coalition with GERB and the latter party should appoint the Prime Minister.

The decision of Parliament's fifth-largest (or sixth-largest, not counting DSB) group came after the two largest parties, GERB and the Bulgarian Socialist Party (BSP), refused to make a try.

Tsvetan Tsvetanov, who heads GERB's parliamentary faction, on Thursday told NOVA TV the chance of a new cabinet being formed was 20%.

His party, however, is ready to take part in government talk as the step is required by "political ethics", the RB having been GERB's junior coalition partner in the outgoing government, Tsvetanov explained.

Asked whether it was important for the UDF, one of the RB parties, that GERB should be at the helm of a new government, Tsvetanov said he was aware a new administration could not be formed without GERB, whose faction has 84 MPs in the 240-seat Parliament.

The anti-corruption bill in particular was one of the main causes of DBG and its leader, outgoing Deputy PM and Education Minister Meglena Kuneva, as she was campaigning in the 2014 election.

However, MPs rejected the bill last year drawing condemnation from EU partners, with the cabinet in resignation publicly promising that, after some amendments, lawmakers would pass it. The new text was adopted in July in a first reading, but the Committee vote prevents it from further debates and a vote at the floor.