Bank watchdogs promised yesterday to investigate allegations that Lloyds TSB was knowingly offering substantial loans to customers who could not afford repayments.

The Banking Code Standards Board, a self-regulatory body representing banks and building societies across the country, said it was taking leaked documents revealed in an investigation by BBC1's Real Story last night "very seriously".

"If we found them guilty of significant breaches we would name and shame them," said Seymour Fortescue of the BCSB.

An investigation by the documentary uncovered evidence that the bank was giving out unaffordable loans. It suggested that the promise of bonuses for hitting sales targets was encouraging staff to pressure customers into accepting large loans.

The BBC programme saw documents from an internal investigation by the bank which showed unaffordable loans were being given out. The audit found that one in six large personal loans were issued despite evidence that the recipients might find it difficult to repay them.

The review examined 185 loans of more than £15,000 and, in 104 of the cases, the paperwork that was supposed to show that proper financial checks had been carried out was incomplete.

The leaked report said: "In many cases it appears the adviser's main priority is to maximise sales points and little attention is paid to the customer's circumstances."

It added that these issues occurred across the whole branch network and concluded: "It is our opinion that some of the issues identified during our review have the potential to expose the bank to significant reputational risk."

The programme heard from a former personal account manager for the bank who said staff received points for reaching targets on the amounts given out. If the account manager did not hit targets "your salary could be reduced and you could eventually lose your job. I regularly saw people being given loans that couldn't afford it."

It highlighted the case of a couple, the Dickersons, who earned just £5,000 a year but borrowed £100,000 over the course of 12 months. Lloyds TSB said it would look at the case but declined to comment further. After the BBC's intervention, the bank agreed to write off the couple's debt.

In a statement, it said it was committed to being a responsible lender. "It is clearly not in our interests to lend money to people who cannot afford to pay us back.