Peers have been accused of treating their House of Lords allowances as "a form of pay".
An independent review of Parliamentary salaries and expenses said some members of the Upper House failed to understand that their role was unpaid.
The disclosure by the Senior Salaries Review Body is likely to raise questions about the amount of expenses currently claimed by many peers.
Members of the Lords can claim up to £239 every day they attend Parliament to cover meals, travel and overnight accommodation in London.
Another £69 a day is available to pay for secretarial support.
Claims for these allowances - which are only supposed to cover the legitimate costs of their legislative role - do not have to be backed up by receipts.
But following its triennial review, the SSRB said the Lords regime was "not clearly understood".
"The basic day and overnight subsistence are flat rate amounts which are not taxable and are intended to reimburse all out of pocket expenses arising from attendance at the House," it said.
"Some peers, however, seem to regard these allowances as a form of pay.
"Others argue that they should be able to claim for additional items where actual expenditure is higher than the flat rate."
