Keir Starmer will not be claiming an official allowance to help cover costs of the PMs clothing and other living expenses.
Downing Street sources say Sir Keir will not be taking advantage of the £20,000 tax deduction - although he has refused to rule out accepting more freebies from donors.
The little-known perk has been around since 1947, although no premier is thought to have accepted it since David Cameron in the coalition years.
The level of the allowance - which is actually a portion of salary paid tax-free - is set by the top civil servant at the Treasury based on expenses they believe are wholly, exclusively and necessarily to the PMs duties.
It is underpinned by the Income Tax (Earnings and Pensions) Act 2003. The cash value of the benefit would depend on the PMs wider income.
It is understood Keir Starmer (pictured at a post-Olympics receptions last night) will not be claiming the PMs official allowance to help cover costs of clothing and other living expenses
The situation emerged as Sir Keir was urged to come clean over who paid for a series of statement dresses worn by his wife Victoria.
As the Wardrobegate backlash raged, the Tories said the PM owes it to pensioners to be honest about who is funding the outfits.
No10 has refused to say who had bought a number of Lady Starmers dresses, although it insisted that everything has been declared as required.
Sir Keir said before the election that his wife doesnt want to push herself into the limelight, but she has worn a number of designer dresses at high-profile events in recent weeks.
Some items were loaned to her, but No 10 yesterday refused to clarify whether the other outfits were part of a £5,000 gift of high-end clothes from Labour donor Lord Alli.
Sir Keir, who earns more than £166,000 a year as PM, was accused of failing to declare on time the donation from Lord Alli, the former chairman of the online fashion retailer ASOS.
Lord Alli has also given Sir Keir suits and spectacles worth some £18,000.
Speaking to reporters on a visit to Italy earlier this week, the PM insisted there was a massive difference between declarations and corruption as he faced questions over his alleged failure to register clothing given to Lady Starmer by the multimillionaire businessman within a designated time limit.
The situation emerged as Sir Keir was urged to come clean over who paid for a series of statement dresses worn by his wife Victoria (pictured at a London Fashion Week event in an outfit thought to have been loaned)
The Tories have demanded a probe by Parliaments watchdog, but according to No10 this has been rejected. Sir Keir argues that he proactively inquired whether he should register the donations.
Although Sir Keir is not accepting the PM expenses allowance, the Starmers are set to be insulated from the impact of energy bill hikes as pensioners lose winter fuel allowance.
The PM and Chancellor only pay a taxable benefit on running costs at the grace-and-favour apartments in Downing Street - capped at 10 per cent of their ministerial salaries.
It means that they contribute around £3,000 to cover all utilities and other expenses, and the sum will not go up when the Ofgem cap increases by 10 per cent next month.