By Rob Cameron BBC Prague correspondent

Mr Spidla made the announcement after coming under severe pressure following the party's disastrous showing in the European elections.

Spidla is accused of alienating left-wing voters

Several days ago, even as a bitter struggle for power was raging within his Social Democrats, Mr Spidla promised Czechs they were in for a quiet summer.

There would be no political crisis, said the prime minister, no early elections.

Most people believed him. Czech politics is rarely dramatic or exciting. Most political dramas have a habit of dying down after a few days and then fizzling out altogether.

But this time Mr Spidla was wrong. The summer now looks like being anything but uneventful.

On Saturday Mr Spidla finally succumbed to a prolonged attack from rebels within his party. They had been gunning for Mr Spidla for some time, as support for the party began slipping steadily in the polls, apparently after alienating many traditional left-wing voters with a series of unpopular public finance reforms.

Sharpening knives

The final straw came two weeks ago, when the Social Democrats were humiliated in the European Parliament elections. The biggest ruling party won less than 9% of the vote, receiving just two of the 24 seats up for grabs.

Mr Spidla's opponents began sharpening the knives, and rumours of a coup were rife.

He was finally ousted at a meeting of the Social Democrats' Central Executive Committee. He narrowly survived a vote of no-confidence - his opponents lost by just six votes. But that vote also made it clear he no longer enjoyed the support of his party.

The expected new prime minister is just 34-years-old

Mr Spidla and his coalition government, which also includes the centrist Christian Democrats and the right-wing Freedom Union, will resign en masse on Wednesday.

The man expected to replace him as both party leader and prime minister, Interior Minister Stanislav Gross, now has the task of building a new government out of the ashes of the old.

Mr Gross has three options open to him. None of them are very appealing.

He could try to reassemble the present three-party centre-left coalition, which has a majority of just one seat in parliament. But following Mr Spidla's removal, the coalition is clinically dead, and Mr Gross knows it would be futile to try and revive it.

Or he could try to create a minority left-wing government, supported by the largely unreconstructed Communist Party. But the Communists are still political pariahs in Czech party politics, and Mr Gross knows such a move would be political suicide.

Untested leader

So the most likely option left open to him is a minority cabinet, made up of the Social Democrats and the Christian Democrats, and supported by the right-of-centre opposition Civic Democrats.

But the Civic Democrats say they will only support such a government as a temporary solution until early elections, something many of Mr Gross's colleagues are keen to avoid. Coalition talks, which begin on Tuesday, will be tough.

However even if he manages to form a new government, questions remain over his qualifications to lead it. At just 34 years of age, Mr Gross - once described as the "crown prince" of the Social Democrats - would be one of the youngest leaders in Europe.

Last year Mr Gross admitted he was not sure he was capable of running his party and the country at the same time. Mr Gross might be one of the country's most popular politicians, but Czechs will have little patience if it turns out he is not up to the job.