The 2011 collapse of one family-owned prime contractor, which left some $130 million in unpaid bills, was the precursor for the region's first PBA project, a municipal fire and police training facility whose contract is about to be signed, says Watterton.

But Bennett says primes were "generally reluctant [to use PBAs] and wanted the status quo maintained."

They are expensive to run and time-consuming to administer, complains Stephen Ratcliffe, director of the U.K. Contractors Group, which represents major firms. "They are not appropriate in all circumstances," he adds.

But PBAs also can prevent contractors from using money owed to subcontractors and suppliers as a source of free credit, says Ian Corfield, a partner with financial consultant Grant Thornton, London. Traditionally, payments made by project owners can "disappear into a series of [contractor] overdraft positions," he adds.

Crossrail tries to protect primes by making payments one month in advance "to keep [firms] in a cash-flow-neutral position," says Morrice. He says a few of the project's prime contractors "struggled conceptually with PBAs, but it has gone very smoothly."

Martin Chown, procurement and supply-chain management director of Balfour Beatty Construction Ltd., says, "We do support PBAs." He sees no evidence of administrative difficulties among supply-chain firms. The use of PBAs is growing mainly on larger public-sector projects, says Chown.

To complement PBAs, Balfour Beatty and Carillion PLC are among prime contractors introducing early payments, in line with the government initiative enacted in October 2012. The voluntary scheme allows subcontractors to be paid early for a relatively small fee.

Under Balfour Beatty's system, launched six months ago, approved invoices are automatically uploaded onto the internet platform of participating PBA banks, allowing suppliers fast-track access to the funds.

The company spent about $170,000 setting up the system, which is self-financing on a marginal or break-even basis, says Chown.

Fees paid by suppliers for the facility equate to an annual interest rate of about 3%, about a third the market rate for some firms, estimates Chown. "We've had suppliers who closed down their credit arrangements to use ours for short-term borrowing," he adds.