Britain is set to be left without an environment watchdog to replace the EU's at the end of the Brexit transition period, because of government "dither and delay" over legislation.

Ministers' flagship Environment Bill was supposed to establish a new enforcement structure for environmental regulations in time for the end of December, when EU rules stop applying.

But repeated delays to the legislation mean there is no longer enough parliamentary time for the bill to pass into law before 1 January – leaving a gaping chasm in environmental enforcement.

Environmental groups said the bill was "the most important piece of environmental law of the last decade" and that the delays would likely lead to extra environmental harm.

"Boris Johnson's government seems to be treating the Environment Bill like an essay crisis," said Rebecca Newsom, head of politics at Greenpeace UK.

"Delaying this vital piece of legislation until the last minute could lead to less scrutiny, fewer chances to improve it and a cliff edge in the environmental safeguards and their enforcement once EU rules no longer apply."

She called for a "powerful, independent watchdog that can hold ministers' feet to the fire when it comes to protecting the living world".

After the resumption of parliamentary business in the summer, bills on immigration, trade, pensions, and the Commonwealth Games were all brought back before MPs – but the environment bill has yet to be so.

The government has extended the bill's timetable three times, and Labour says the latest schedule means MPs will not be able to give it detailed scrutiny until 1 December – the start of a process that takes months.

Luke Pollard, Labour’s shadow environment secretary, said: "The government’s dither and delay highlights a worrying lack of focus on the urgent need to protect our environment. We cannot afford any more government incompetence. Ministers need to act now."

The government says it is committed to resuming passage of the bill. A Defra spokesperson said: “We are committed to a greener future, which is why we are setting ambitious goals for nature and biodiversity in our landmark Environment Bill, as well as introducing new ways to reward farmers for protecting the environment and investing £640m in the Nature for Climate Fund.

“We remain the first major economy to legislate for net zero, and as we build back greener from the coronavirus pandemic we are committed to shaping a cleaner and more resilient society.”

The UK left the EU earlier this year, but the Brexit transition period under which all EU rules continue to apply to Britain is set to end on 31 December. The EU and UK are trying to negotiate a free trade agreement before this date to facilitate trade.