London transport bosses will meet Treasury officials on Thursday to discuss extra funding for the £15.4bn Crossrail project, after a delay to its opening was announced last week.

The overrun on the central section could approach a year, it has emerged, with the revised target for the opening of the new Elizabeth line now the end of autumn 2019 instead of December this year.

The mayor, Sadiq Khan, said he was “keen to ascertain costs” arising from the construction and testing problems on Crossrail, whose revenues are a key part of Transport for London’s business plan and Khan’s pledge to freeze fares.

Significant additional construction costs are expected as well as hundreds of millions in lost revenues. Although TfL has so far forecast only £20m in net revenues would be lost up to March 2018-19, when users of the new Crossrail central section would mostly have been regular tube passengers, delays to the full route would spell far greater losses.

The full route is expected to bring in more commuters on direct trains, running across London from Reading to Shenfield. Direct trains had been planned to run from London Paddington to Essex next May, and across the entire route in December 2019.

Khan told the London assembly: “I’m extremely frustrated, disappointed and angry at the delay but I’m confident that when completed Crossrail will be a great engineering project and asset for London.”

The mayor said Crossrail had been discussed at every TfL executive meeting since 2016, due to the independence granted to the Crossrail board, which reports to both TfL and central government. Yet he had not been informed of the delay until a review on 29 August, two days before the postponement was announced.

“At no point till recently were we told that the opening date for the central section would be breached,” Khan said.

The transport commissioner, Mike Brown, said that the full extent of further costs were yet to be ascertained, but he would be meeting the Department for Transport and the Treasury on Thursday “to explore potential challenges and how we can address them”.

In July TfL and the government injected a further £600m to the project, which had long boasted of being on time and on budget.

Questioned about when TfL was informed of a possible delay, Brown said that at a TfL board meeting in July “there was a concern expressed but nothing categoric”.

Crossrail’s chair, Terry Morgan, rejected claims he had concealed the extent of problems from politicians, having addressed a meeting of MPs in late July, four days after a board meeting when a fresh review of the feasibility of a December opening was commissioned.

Morgan admitted he had assured politicians inspecting Crossrail works in June that they would meet an opening date of 9 December, despite the problems.

Crossrail’s chief executive, Simon Wright, said problems with the electrical and signalling systems had delayed testing of trains for four months but they had tried to maintain the schedule.

Morgan said: “We found ourselves in a situation where when we mitigated one risk another appeared … A combination of delays on construction and the complexity of testing the new systems, and a lack of productivity of testing, led us to that decision [to postpone the opening].”