The government has expressed concerns about the timeline for implementing assisted dying, the BBC understands, before measures were tabled to delay when it could be available.

Kim Leadbeater, the Labour MP bringing the bill, said she was “disappointed” to be proposing deferring when it must be in force by until 2029.

Some MPs expressed concern this was the same year as the next general election was due.

Officials and civil servants have been advising Leadbeater on drafting amendments, including suggesting changes the government wants.

A spokesman for Leadbeater said: “Kim hopes and believes the service can be delivered more quickly if it becomes law later this year.”

Part of the reason for postponing the deadline is to allow time to set up training and systems for what would be an entirely new service. Changes made to the bill since MPs voted in favour, including a new commission and panel system to oversee application, have added to that.

Leadbeater’s new amendment changes the maximum implementation period from two years to four years. However if all the systems are ready before four years, assisted dying will go ahead earlier than the deadline.

It is understood there is disagreement among ministers about the delay to implementation, with some believing it is a mistake.

The government is officially neutral on the bill with many ministers supporting it.

“I cannot pretend that I’m not disappointed about extending the commencement period,” Leadbeater told MPs on the committee tasked with scrutinising it.

But she said the change created a “four-year backstop”, adding: “It doesn’t say it cannot be implemented before then.”

Both MPs supportive of assisted dying and those opposed have expressed dismay.

Tom Gordon, a Liberal Democrat MP in favour of the bill who sits on the committee, said other countries had implemented similar laws in much less time.

“The current law is failing terminally ill people, forcing them to suffer unbearably, travel abroad to die or risk their families facing police investigations. This amendment would prolong that suffering unnecessarily,” he said.

“Delaying implementation risks pushing it beyond the next election, where it could be abandoned altogether.”

Jess Asato, a Labour MP who is against the bill, said: “Putting an arbitrary timeline on such a complex issue was never a sensible way to make life or death legislation. This last-minute switch demonstrates again just how chaotic this whole process has been with substantial last-minute changes to core sections of the bill.”