The Bank of England is scrutinizing allowances awarded to top staff by banks in an effort to establish whether they are a covert way of avoiding a new European Union cap on bonuses, a senior official at the central bank said on Tuesday.

From next year bankers' bonuses in the 28-country EU can be no higher than fixed salary, or twice that amount if a bank's shareholders give their approval. However, HSBC, Lloyds and Barclays are all considering giving top staff monthly or quarterly allowances to boost fixed pay.

About 80 percent of the bankers affected by the rule are based in London.

The Bank of England is toughening its stance on excessive compensation in response to public and investor criticism of a bonus culture blamed for contributing to the financial crisis and has asked its supervisory arm, the Prudential Regulation Authority (PRA), to study the proposed allowances.

"We haven't looked at role-based allowances in enough detail to be able to give a house view on whether they comply or not. Our job is to work with the European authorities," Katharine Braddick, a director of policy at the PRA, told reporters on the sidelines of a financial conference.

The European Parliament, meanwhile, is considering whether its bonuses rule needs tightening and the European Banking Authority has asked the PRA to confirm whether proposed allowance payments comply with the cap.

Braddick said that the PRA will have to gauge whether the role-based allowances are what they say they are or merely a means to skirt the bonus cap.

The Bank of England is due to publish a consultation on Thursday on a proposed rule to allow the clawing back of bonuses up to six years after they are paid, Braddick said.

The change would require banks to rewrite staff contracts from January 2015.

The aim is to punish bankers whose behavior is later proven to have put a bank at severe risk or whose conduct turns out to have fallen short of the standards required.

Bankers would be legally liable to return the money, if requested, even if they have left the bank.

"You can't claim you've never had it. If you've had it and you've spent it and you can't get anything back, then you have to think about how you would liquidate some of your other assets," Braddick said.

Another measure likely to be considered is a lengthening of deferral periods for bonuses. Most of a bonus is deferred for three to five years, but some UK lawmakers have called for this to be extended.

Braddick said the PRA believes there is a strong case for longer deferrals.

There is already a rule to allow a bank to claw back those parts of a bonus a banker has yet to receive.

(Editing by Jane Merriman and David Goodman)