It's a story of six-figure kickbacks, briefcases of banknotes handed over in car parks and politicians pocketing cash-filled envelopes that allegedly goes right to the top of Spanish politics. And yet the man who has the most explaining to do, Spain's prime minister, Mariano Rajoy, refuses to discuss the allegations, which he dismisses as lies and insinuations.

Despite a series of well-documented allegations that Rajoy and senior party members received illegal cash payments over a period of years, Rajoy seems prepared to tough out growing calls for his resignation and the threat of a motion of censure, knowing that parliament and the nation are about to pack up for the summer.

However, the questions will still be waiting to be answered when Spain wakes up again in September. Alfredo Pérez Rubalcaba, the opposition Socialist leader, said on Sunday that "until he answers these questions, Rajoy cannot govern".

According to a survey published in the conservative El Mundo newspaper, 83% of Spaniards believe the allegations and think Rajoy should answer them. However, Rajoy's Partido Popular (PP) has an overall majority and even Rubalcaba admits the motion of censure is largely symbolic.

"Rajoy's strategy has always been the same. He never wants to explain or justify his policies," says Antonio Argandoña, a professor of business ethics at IESE Business School, speaking in a personal capacity. "It would be difficult to change now. I think he's counting on sticking it out until everyone comes back from holiday; there'll be something else on the front pages. He may even take big and unpopular policy decisions to show that he can stand the pressure and won't be blown off course."

The scandal broke in 2009 when the alleged slush fund was investigated. The anonymous donations came from large companies, mainly in the construction industry, in order to sweeten deals for public contracts. One leading politician was allegedly paid in order to secure the refuse collecting contract for the city of Toledo. The allegations against the PP's treasurer over a period of 20 years, Luis Bárcenas, first emerged in 2009 but the story took off at the beginning of this year when El País newspaper reproduced the accountant's handwritten records detailing illegal monthly cash payments to senior politicians, including €250,000 (£215,000) to Rajoy. According to Bárcenas, senior party figures were paid an "extra salary" ranging from €5,000 to €15,000 a month in cash. Under Spanish law, government ministers may not receive any other income apart from their government salary.

It is also alleged that the party's secretary general, María Dolores de Cospedal, took a €200,000 kickback that was handed to her in a briefcase in a car park. The ledgers also record slush fund payments of several thousand euros to the current health minister, Ana Mato, allegedly to cover the cost of her children's communion and birthday parties.

Bárcenas is in prison on remand, having been declared a flight risk after it was revealed that the former accountant has €47m in Swiss bank accounts.

The government at first dismissed the El País documents as mere photocopies and stood by Bárcenas, who initially denied authorship of the ledgers. This month El Mundo published the originals and last week Bárcenas admitted they were authentic. The PP has now turned its back on its former treasurer, calling him a criminal and a liar. However, last week El Mundo published text messages sent between Rajoy and Bárcenas that show that as late as March Rajoy was expressing his solidarity with Bárcenas and urging him to hold his nerve and keep quiet. Now the PP fears Bárcenas, angry that his party has abandoned him, may have more up his sleeve, including recorded conversations that will implicate Rajoy.

The Rajoy allegations are the latest in a succession of corruption cases, of which there are more than 200 currently before the courts. They involve politicians ranging from village mayors to former cabinet ministers, as well as leading business and cultural figures and even the royal family. Spanish people knew the system was bad, but few imagined it was this bad. Transparency International, which assesses countries on their perceived levels of corruption, ranks Spain 30th, just below Botswana and one place above Estonia, out of 176 countries surveyed.

If people are not taking to the streets in any numbers to demand Rajoy's resignation it is because they are disillusioned with the political system as a whole. Rubalcaba scores even lower in approval ratings than Rajoy, who himself barely makes it into double figures in most surveys.

"The positive thing about a crisis is that it exposes the hidden reality and this is what's happened in Spain," says Francesc de Carreras, who teaches constitutional law at the Universitat Autònoma in Barcelona. "Economic growth was partly based on false foundations. The government has set off on a new economic path but what hasn't begun is the necessary reform of political parties."

At the moment Rajoy is looking no further than the end of the week. He has no immediate rivals either within his party or in opposition. However, the scandal has attracted international coverage and fear of political instability is damaging confidence in the country. "I think Rajoy will survive all this but Spain's image right now is very bad all over the world," says Argandoña.