Image copyright Christopher Furlong Image caption The "masterplan" aims to set out a comprehensive policy framework for the environment

The government's long-delayed 25-year plan for improving nature in England should be published immediately, MPs have said in a letter to the Environment Secretary Andrea Leadsom.

They asked her to explain why the strategy due in 2016 is still not out.

They say it's essential that ministers have agreed and published a clear plan before negotiations on Brexit begin.

The government promised in its manifesto that it would leave nature in a better state than it was inherited.

The all-encompassing masterplan aims to set out a policy framework for air, freshwater, marine, wildlife, soils, flooding, forests, and even the urban environment.

Environment charities which have been briefed on the plan say it is appropriately ambitious in some respects - but they, too, want to see it in black and white.

Long-awaited

The plan was originally due to be published in summer last year. I understand that the document has been signed off by the Prime Minister but has been delayed for weeks, waiting for the ideal time to publish.

The letter of complaint to government has been signed by all the members of the Environment Audit Committee, which involves Conservative, Labour and Green MPs. The Lib Dems told BBC News they supported the demand.

Their letter says: "We welcome this important step by the government to take a longer-term approach to protecting our environment. However, we are disappointed by the continuing delays in publication of the framework.

"First, the framework (of the plan) was delayed from summer last year to the autumn following the referendum. We are now in March, the framework has still not been published and there is no indication of when it will be."

The letter accepts that Defra has been under strain from preparations to leave the EU, on top of budget cuts. But it says: "The Plan should be published and consulted on before Article 50 is triggered, so as to inform the government's negotiating position.

"This seems unlikely, raising the prospect of the government entering crucial and time-limited negotiations with the EU without an agreed plan. It is essential that the 25-year plan is not delayed further."

Members are particularly concerned about post-Brexit policy on farming, which will have a huge impact on wildlife, water and air pollution, soil loss and flooding.

They want assurances that when the UK is no longer under the jurisdiction of the Commission and the European courts, the government will still be able to be held to account if it fails with its environmental promises

The government is preparing a separate but related plan for post-Brexit farming. I understand this is also finished and approved but awaiting a "suitable" publication date.

Peter Morris from the Wildfowl and Wetlands Trust told BBC News: "We're hoping for a legally-binding plan, with milestones for a healthy environment, the money to invest in our natural world, and clear monitoring to make sure it succeeds. This should be backed up with a commitment for a new Environment Act to establish the plan in law."

A spokesman for the government told BBC News it would publish both plans as soon as possible.

Restoring nature

The government wants to leave Nature in a better state: here's three examples of how that can be done.

Image caption Chris Miller from the Wildlife Trusts is seen here with baby bog mosses

Carbon stores: Peat bogs are good for wildlife and flood prevention - and for storing carbon (they store 10 times as much as England's forests, say the Wildlife Trusts). Yet 80% of the UK's bogs are damaged or lost. On the western edge of Salford, conservationists are cultivating sphagnum moss in polytunnels to re-colonise the devastated landscape.

Image caption Farmer Steve Honeywood stands in a woodland ride full of birdsong.

Suffolk hedgerows: Hedges are a rich habitat and a haven for pollinators but over 100,000 km are estimated to have been lost between 1984 and 1990 alone. In Suffolk one farmer, Steve Honeywood, has seen bird numbers treble and species increase from 60 to nearly 90 by measures including pruning just one edge of his hedges each year and leaving two uncut edges for wildlife. He's now planting elm hedges to support the elusive white-letter hairstreak butterfly.

Image caption Wildlife worker Helen Spring in the community garden in South London

Urban wildlife: In South London, locals are working to create green areas along the catchment of the lost River Effra, which was turned into a closed sewer in Victorian times.

By smashing up concrete and tarmac they want to extend flood resilience and improve neighbourhoods for people and wildlife. The government wants to extend people's contact with nature.

Follow Roger on Twitter.