The new 2021 target for the opening of Crossrail is liable to slip further due to continuing uncertainty about the software behind the new rail line, its chief executive has admitted.

Crossrail on Thursday confirmed reports last week that services in central London would not start until as late as March 2021.

A six-month window from autumn 2020 is the new target, the first public commitment since the December 2018 opening was abandoned. Crossrail said it was “a robust and realistic plan” to put the project back on track. But it warned that Bond Street, one of the main central stations, would not be ready to open by then.

Hundreds of maintenance workers hired for Crossrail will be paid to “practise” until services start, it has emerged.

Unions said the plan to start services in central London could mask further delays to the full running of the £17.6bn rail line, integrating the central and eastern sections. Twelve trains an hour will run across the central section when it opens, rather than the 24 trains an hour eventually planned.

The Crossrail chief executive, Mark Wild, who was brought in to review the project when the original management team departed as problems emerged, said the date remained uncertain because engineers still did not know what they would find in testing.

“This project is of immense engineering complexity,” Wild told the BBC Radio 4 Today programme. “The work we’ve done is working out the very careful sequence of knitting together all these very complex systems.”

He said that while they were testing trains at full line speed in tunnels under London, and hoped to complete much of the infrastructure in the months ahead, intensive testing would be needed in 2020. “We don’t know what software bugs we’ll find in that phase and there’s no short cuts,” he said.

He gave no assurance that it would definitely be open by 2021: “Our uncertainty is simply that when we get into proper train reliability growth next year, we don’t know what we’ll find.”

Wild said that if there was a lesson for HS2, the high-speed rail network that will be Britain’s next transport infrastructure megaproject, it was that as a “digital installation” there were different risks from other engineering works. “When you get into software development and bringing that to fruition, there is obviously uncertainty and people should be honest about that,” he said.

Wild also admitted that hundreds of staff were not doing the work they had been hired to do in anticipation of services starting. Speaking to LBC, Wild said that while 500 train drivers were “fully utilised” in testing and operating Transport for London rail services, “the drivers are not actually the significant issue. The issue we have is our maintenance technicians. There are about 200 highly trained maintenance technicians who are practising.”

He said it was “frustrating for them and disappointing”.

On Thursday, the TfL commissioner, Mike Brown, told the London assembly that he would not quit, despite being urged to consider his position. A transport committee report found he had “watered down” the problems and risks of delay at Crossrail in communications with the London mayor, Sadiq Khan.

Transport unions also doubted the new target dates. The TSSA general secretary, Manuel Cortes, said it was more “jam tomorrow” from Crossrail: “Londoners who are footing the bill for this delay will not be fooled – there’s nothing concrete here about when full services will commence.” He called for a public inquiry into a “fiasco which has seen private contractors carry on making millions despite their failure to deliver”.

Crossrail’s budget was blown last year, with two bailouts and contingency loans bringing the total bill to an expected £17.6bn, almost £3bn more than planned.

The Elizabeth line, as Crossrail will be known, will eventually link Heathrow and Reading in the west of the capital to Shenfield and Abbey Wood to the east, and carry 200 million passengers a year.