Businesses throughout Europe are struggling to collect an unprecedented mountain of unpaid invoices as customers withhold payments in order to conserve cash in the face of coronavirus-related lockdowns that have sent the world economy into a sudden deep freeze.

The amount of money that is more than 10 days overdue has grown 56% in France, 52% in Spain, and a whopping 82% in Italy since the World Health Organization declared a pandemic on March 11. The U.K. and the Netherlands have each seen about a 25% increase, while Belgium has seen a 44% spike.

The picture is even more grim when looking at the total number of invoices that are now 10 days or more past due. That figure has soared 70% across the continent, with the largest increase—a near doubling of the number of unpaid bills—happening in the U.K., followed closely by France, which has seen an 87% increase in late payments.

That's according to data released on April 22 by Sidetrade, a Paris-based software company that helps businesses with customer engagement, cash management, and invoice collection. The firm looked at data for some 28.5 million business-to-business invoices, adding up to more than 50 billion euros ($54 billion) and involving about 4 million companies. It has created a tracker to follow invoice payment data for the six European countries.

Mark Sheldon, Sidetrade's chief technology officer, said the sudden leap in unpaid invoices is "unprecedented" and can't be easily compared to other previous recessions.

Businesses are acting in their own interests to conserve cash by delaying or withholding payments. But by not making payments in a timely manner for products already received or services rendered, these companies are simply passing pain on to their suppliers and other vendors. Combined with the fact that little new business is being generated during the lockdown, the invoice collection problems present a double whammy for many businesses.

Spluttering cash flow can have a domino effect throughout the economy, with smaller companies often the first to collapse. In the U.K., as many as one out of every five small businesses were in danger of failing by May and were struggling to access emergency cash from the government in time to stave off that fate, according to a report from the Corporate Finance Network, a trade body for accounting firms and consultants who service small and medium-size enterprises (SMEs).

Even in normal times, smaller businesses often have trouble collecting invoices from larger corporations, as the bigger company often uses its market power to extract concessions from the smaller firm. "B2B payment has always been a pain point from SMEs, and it is being made significantly worse by this crisis," Sheldon said.

He said that many companies are having to be "innovative" about the way they try to collect payments. More have begun offering discounts to encourage on-time payment, for instance.

Other tactics to successfully collect payment can include changing the methods a company uses to pursue unpaid invoices, Sheldon said, as well as altering the payment channels it will accept. For instance, email and other digital forms of communication are likely to be more effective than phone calls when most employees are working from home, and switching to a subscription model for services may result in more reliable payment.