Andy Byford, the Transport for London (TfL) chief who helped steer the capital's bus and Tube networks through the most financially precarious periods in their history, has resigned weeks after securing a new long-term funding deal.

Sky News can reveal that Mr Byford's departure as TfL commissioner will be announced on Thursday morning.

His exit after nearly two-and-a-half years in the job will leave the London mayor, Sadiq Khan, searching for a successor who can build on the outgoing chief's legacy.

An industry source said on Wednesday evening that Mr Byford had notified the mayor of his intention to step down several months ago.

He is expected to leave before the end of the year.

The TfL commissioner's tenure saw him deliver the Elizabeth Line project - more commonly known as Crossrail - after years of financial and operational problems.

The 73-mile line is now carrying hundreds of thousands of passengers every day.

Mr Byford's most important short-term achievement, however, was the delivery of a long-term funding deal for TfL after two years in which its balance sheet was ravaged by the COVID-19 pandemic.

Caught in the crossfire between Grant Shapps, the then transport secretary, and Mr Khan, the TfL chief was forced to manage the bus and Tube networks on short-term government handouts which sometimes lasted only a matter of days.

The organisation narrowly averted having to file formal bankruptcy notices on several occasions, such was the fraught nature of the negotiations.

Its latest deal, finalised late last month, runs until March 2024.

In total, TfL has been handed ?6bn to keep it afloat since the outbreak of COVID in the spring of 2020.

On Wednesday, Mr Khan said he would provide an additional ?500m of funding to TfL, triggering suggestions of council tax hikes for inflation-hit Londoners.

The London mayor had also warned of significant cuts to services prior to the latest deal being struck, while he said last month that a "significant funding gap" remained and would likely mean fare increases.