Mr Lepper announced he would pull his two ministers out of the cabinet immediately after the summer break ends (22 August), while the party's 46 deputies in the 460-seat parliament will no longer support the government's policies.

The latest political crisis comes in response to a row between Mr Lepper and prime minister Jaroslaw Kaczynski over who should be appointed an agricultural minister in the cabinet, but the tension between the two has a long history.

Last month, prime minister Jaroslaw Kaczynski sacked Mr Lepper, then a deputy prime minister, over a major corruption case involving bribes. Mr Lepper insists he is innocent.

In September 2006, a dispute over the budget also saw the Self Defence Party quitting the coalition, but three weeks later it was invited back because of risks linked to an early election.

Without the Self Defence Party's support in the parliament, prime minister Jaroslaw Kaczynski will have to count on the support of only his own Law and Justice party and the right-wing nationalist League of Polish Families – in total 203 deputies, far short of the 231 needed for a majority.

In addition, the voters – if called to the polls this autumn, two years ahead of schedule – are expected to support the left-wing and liberal opposition. The regular election was due in 2009.