Mayor of the Lääne-Viru County town of Rakvere Triin Varek is one of the latest defectors from the Center Party, to Isamaa, citing the "ambiguous" leadership of Mihhail Kõlvart.
The move spells the end of the Center-Isamaa coalition in the town of around 15,000 people, but an Isamaa-led administration says it will continue with policies and projects agreed in the earlier coalition agreement.
Varek said of her decision that she was "grateful to all the voters, supporters and party colleagues for the past six years, but unfortunately I do not see an opportunity to continue in the ranks of the Center Party in a situation where the course of the party is unclear. I have informed the Center Party group at the city council of my decision, and they subsequently expressed their desire to move forward, together."
With Varek joining Isamaa and three council deputies, Erika Bauer, Jaak Vettik and Liidia Bobkova also leaving Center, a new governing agreement was required – one has been forthcoming.
Additionally, Sivar Irval, deputy council chair who ran for Center at the last local elections but was not a party member, announced he would be joining the Isamaa grouping of councilors.
"Our cooperation with Isamaa had been constructive and effective, and bearing the interests of the city in mind, everyone wants to continue on this course. This is a value-based decision, while it is not insignificant that the agreements reached two years ago will remain in place and the interests of the voters will thus be safeguarded," Varek said.
Ain Suurkaev, who is head of Isamaa's Rakvere branch, said: "We are delighted that our current coalition partners at council level are ready to continue working together. This shows mutual trust, and a willingness to continue to contribute to shaping the present and future of the city of Rakvere, as active deputies. It will not be easy at present and in the near future for governments at either state or municipal level, so let's not complicate our lives, either."
Isamaa now has 12 seats on the 19-seat council and has pledged not to go back on earlier pledges made in the coalition with Center.
Center and Isamaa are both in opposition at national level; other high profile defectors from Center to Isamaa in recent weeks include MPs Jaanus Karilaid and Tõnis Mölder.
Mayor of Tallinn Mihhail Kõlvart's ascension to the position of Center leader has prompted fears among some party members of a stronger emphasis on the Russian wing of the party versus the Estonian wing.
Editor: Andrew Whyte, Mari Peegel