Ms Kallas specified three points where coming to an agreement with Prime Minister Jüri Ratas' Centre Party would be very difficult, and where she wouldn't compromise.

No progressive income tax in any shape or form

Tax policy is one of those points: the Reform Party wants to see income tax, currently geared towards granting high income earners a smaller tax-exempted income or none at all, return to an earlier state. According to Ms Kallas, €500 per month should be granted to all Estonian residents tax free, without any additional modification to how income tax is levied.

The argument behind the current coalition's modification of the income tax system is that without such measures, Estonia's approach to taxation is regressive, as with a high-income earner's increasing disposable income, the actual amount of taxes paid shrinks, with the result that low-income earners pay a larger share of their money to the state.

The modified tax-exempted income compensates for this, granting a greater amount of tax-free money to low-income earners while those earning more also have to pay more tax. The measure amounts to a de facto progressive income tax, a concept to which a large bloc inside the Reform Party is still very much opposed.

Citizenship policy to remain on present course

Another issue where it would be difficult to find common ground is citizenship policy. Ms Kallas stressed that the Reform Party wants to hold on to the current system, under which any applicant for citizenship needs to pass a language exam.

While Mr Ratas' government and party do not aim to abolish the language exam as such, it would be open to some forms of expedited naturalisation, especially in the case of people of families who have resided in Estonia for an extended amount of time, such as certain groups of holders of the Estonian alien's passport.

Estonian-language instruction starting in nursery school

The third great obstacle on the way to a Centre-Reform coalition is the former's approach in the currently ongoing education debate. Where the Centre Party is leaning towards keeping up Estonia's Russian schools, the Reform Party wants an entirely Estonian-based education system, starting as early as nursery school.

While both sides have played at ethnic differences and matters of culture and cultural autonomy, it is a confirmed fact that plenty of Estonia's native Russian speakers, particularly families, aren't opposed to such a step. The debate revolves around how the problem should be solved in practice, and concerns a few very real issues, such as the lacking Estonian language competence of hundreds of teachers at Russian schools.

Kallas: EKRE still only party Reform won't work with

"These are the three big issues. The rest of the topics can be negotiated. But in matters of principle, we won't compromise," Ms Kallas said.

Though some in Ms Kallas' party, eg former Minister of Economic Affairs Kristen Michal, have hinted that they would be happy to entertain the thought of a possible coalition with the Conservative People's Party (EKRE), Ms Kallas reiterated that EKRE is the only party with which she won't negotiate and which is out of the question as a potential coalition partner.