Sadiq Khan's estimate of the cost of freezing fares for four years is "perfectly legitimate" the chief of Transport for London said today.
Mike Brown has previously said Khan's commitment would cost £1.9 billion, well in excess of the £450 million estimated by Labour's mayoral candidate.
However, when asked today about the difference between the two calculations, Brown said the much lower projection made by Khan was understandable.
"The way this has been calculated, I absolutely can see why the calculations are different," he told LBC.
He explained that TfL's initial estimate was based on a set of inflation and business plan assumptions that may not arise.
"If [you use] the inflation assumptions we have used from the Office of Budget Responsibility, GLA economics and Bank Of England… then you get to our number. But it is perfectly legitimate... it is perfectly conceivable that people will use different assumptions.
He pointed out that TfL's £1.9 billion figure was based on a five year business plan, while Khan's figure is based on a four year mayoral term. He also acknowledged that TfL's calculation rests on the assumption that fares will continue to rise above inflation, despite the fact that they have not done so over recent years.
"If you start from a baseline of zero fares increases then you come to a different number, so there are perfectly legitimate ways why the calculation has turned out differently," he added.
Khan originally denied that TfL had ever made an estimate of £1.9 billion and accused journalists who reported that they had of "misleading" Londoners.
Brown's comments today will be met with relief from Khan, who has continued to face questioning from the press and his political opponents about the "black hole" in his fares policy.
They also come as a poll this week suggests Khan remains on course to be the next London mayor and the overall head of TfL.
Asked whether he would be happy to work under Khan, who has repeatedly described the organisation Brown manages as "flabby," he replied: "I look forward to whoever is elected to be mayor of London. They will be my boss and I am excited at the prospect."
A spokesperson for the Conservative candidate Zac Goldsmith said Brown's comments only "confirmed" there is a black hole in Khan's plans. .
"Mike Brown confirmed that TfL's inflation assumptions have been taken from a variety of authoritative sources including the Office of Budget Responsibility, GLA economics and the Bank of England," they said.
"If Khan doesn't think the Bank of England is a good source on economic data, then we can only assume he thinks Jeremy Corbyn is instead."