Crossrail could now cost almost £3bn more than budgeted, and the opening of the rail line across London is set to be further delayed until at least 2020.

A fresh bailout announced on Monday by the mayor, Sadiq Khan, and the Department for Transport (DfT), includes loans of up to £2.05bn to London. It means the final bill for Crossrail could reach £17.6bn, instead of the £14.8bn it was expected to cost as recently as June.

A review of the project by the new Crossrail chief executive, Mark Wild, has found that the planned opening in autumn 2019 – postponed just over three months ago from the original date of 9 December 2018 – is no longer viable. A new opening date has yet to be set.

Crucial parts of the infrastructure for the east-west rail line across the capital, including stations and tunnel interiors, have yet to be completed. The complex signalling system will also need extensive safety and reliability testing.

Reviews by the accountancy firm KPMG of Crossrail’s financing and governance arrangements, commissioned by Khan and the Department for Transport, found the delay to completion would be likely to cost between £1.6bn and £2bn.

After the DfT and Transport for London jointly chipped in an extra £300m in July, the government department will loan the Greater London Authority £1.3bn and a further £750m to TfL for contingencies in the latest bailout. The GLA will also contribute an extra £100m cash.

“It has been increasingly clear that the previous Crossrail Ltd leadership painted a far too optimistic picture of the project’s status,” Khan said. “I have ordered the release of all Crossrail board minutes in the last five years to provide transparency to Londoners on their decision-making, and working with the DfT, brought in a new leadership team.

“With London’s population continuing to grow, our priority must be getting this monumental project completed as soon as possible, with Londoners enjoying all the benefits the Elizabeth line will provide.”

The documents show that Crossrail executives had flagged up that its opening date was at risk from July, but weekly official briefing notes to the mayor as late as 21 August continued to state that central London services would start on 9 December.

Tony Meggs, the chief executive of the Infrastructure and Projects Authority, was due to be ratified as the new chair of Crossrail Ltd after Sir Terry Morgan quit last week.

TfL said that it was only just becoming clear how much work was left unfinished on the flagship infrastructure project, whose leaders had long boasted that it was being constructed on time and on budget.

Wild said it was “evident that there is a huge amount still to do”. He said his team was “working to establish a robust and deliverable schedule” before confirming a credible opening date.

The National Audit Office is planning to investigate Crossrail. Khan said the KPMG reports and minutes of board meetings would all be published in the interests of transparency.

The Elizabeth line, as Crossrail will be known, should eventually carry more than half a million passengers a day, linking Heathrow, the West End, Canary Wharf and beyond.