They were elected on the promise of delivering prosperity to Argentina, but statistics showing a stunning economic turnaround have come with a catch.

New figures show that since Nestor and Cristina Kirchner came to power in 2003, they have presided over a remarkable sixfold increase in their own wealth.

The couple have racked up a fortune through property speculation and investments that have thrived even as the economy has faltered. Last year alone their wealth jumped 158% to £7.3m.

Opponents have accused the Kirchners of exploiting political connections in their home state of Patagonia to buy municipal land cheaply and sell it at a vast profit. “It’s a scandal,” said Patricia Bullrich, a member of congress.

The couple, lawyers by training and leftists in the Peronist movement, denied any wrongdoing and through a spokesman said that being in office did not impede business deals: “That is the essence of capitalism.”

In an unusual tandem, Nestor served as president until 2007 when he stood aside for his wife, a veteran senator and politician in her own right, who was elected in the first round over a divided opposition.

They were popular for presiding over a speedy recovery after Argentina’s econnomic meltdown in 2001-02. But underlying problems became apparent after “Queen Cristina”, as she is known to some, took over.

Analysts said inflation was perhaps triple the official rate of 9%, a figure widely viewed as a product of government fiddling, and a bruising battle with farmers over export taxes was compounded by a drought. After six consecutive years of steady growth the IMF expects GDP to shrink by about 1.5% this year. Industrial activity has slumped.

With their own party riven by in-fighting, the Kirchners lost control of congress in mid-term elections last month. In their Patagnonian fiefdom, however, they have notched up property deals that would have made Donald Trump proud.

According to information the couple supplied to the anti-corruption office, they own 28 properties valued at $3.8m, four companies worth $4.8m and bank deposits of $8.4m. Last year they sold 16 properties, almost tripling their bank accounts, and expanded their hotel business in El Calafate, a tourist magnet. Their debts also jumped because of bank loans.

Local authorities have investigated transactions over suspicions that a mayor had given the Kirchners a bargain price for municipal land, but the case has stalled.