Recycling rates at one of the North London Waste Authority’s two plants are amongst the worst in the country – meaning a fifth of the waste sent there is burnt rather than recycled.

Two material recovery facilities (MRFs) are run on behalf of the NLWA by Biffa and Bywaters, where all the plastic, paper, cardboard, glass, aluminium and steel is sent from Hackney, Islington, Barnet, Camden, Haringey, Enfield and Waltham Forestto be sorted and sold on.

But last year Bywaters, which was paid £1.7m to run a plant in Leyton, sent 20pc of that to be burnt at the NLWA's incinerator in Edmonton - adding to the toxins in London's already filthy air.

Biffa was paid £3.8m to run its Edmonton plant, and last year 11pc of what it processed was also burnt. They face no penalties for not recycling the recycling.

On average nationally 5 per cent of recycling was burnt or put into landfill last year. The worst offenders were Barrow-in-Furness where 46pc was rejected, Newham where the figure was 33pc, South Holland at 26pc and Boston at 21pc.

Biffa was fined nearly £600,000 by the Environment Agency last month for trying to export dirty nappies and dog mess in prime paper recycling in 2015, which had been sorted at the north London plant.

Greenpeace's investigative journalist team Unearthed and the Telegraph sent a reporter undercover to work for six weeks at the Western Riverside Waste Authority, which sorts waste for west London, and claims to have witnessed workers throwing unopened recycling bags into a pile to be burnt - leading to accusations that the recycling wasn't being properly sorted through. Here 13 per cent of the recycling is rejected as waste.

Bywaters and Biffa declined to comment but the NLWA claims its figures are as a result of contamination of the rubbish, and blamed people for putting stuff in their recycling that shouldn't be there.

"It is simply not the case that recyclable material sent to the MRFs in north London is not being recycled," said a spokesperson.

Pat Venditti, programme director at Greenpeace UK, told the Gazette: "No one thinks when they put their recycling out that up to a fifth of it will end up burnt or in landfill. The solution is obvious - we need to massively reduce the amount of plastic and other waste being used in the first place, and then ensure what is meant to be recycled ends up just that."

The Gazette asked the NLWA to visit one of its MRFs. They declined, saying they were booked up until November.

A spokesperson said: "We are pleased that residents are so engaged and interested, and recognise how important it is for residents to see for themselves what happens to their recycling. However, the MRFs operate 24 hours of the day, managing the recycling of their other customers as well as that of north London, and the tours take time and resources to organise."

When the Gazette asked to visit in December we were told they weren't yet taking bookings because by then the same firms would be operating under new contracts, and that we could be added to a waiting list. Green party London Asembly member Caroline Russell commented: "The NLWA should be proud of its MRFs, and if they aren't and they are putting off a visit from a journalist it makes it seem as if they aren't proud of what they are doing, and that's really worrying."

She slammed the recycling processing figures as "disgraceful".

"Given the current terrible recycling rates there is already far too much waste going to incineration, and this again is just unimaginably complacent," she said.

"The stuff that is being burnt includes plastics, and burning fossil fuels should have no place in a city that's declared a climate emergency. We should be managing our waste much better, and it should be the absolute last resort to burn it."

Two million people in London live in areas with illegal levels of air pollution.

She has worked out it will take over 100 years for boroughs to meet their recycling targets at the current rate.

Mayor of London Sadiq Khan wants to make London a zero waste city and has set a target for each borough to recycle 70pc of its waste by 2035 - but recycling figures have decreased in the last few years, and while the UK average is 43pc, London is 33pc. Hackney's recycling rate is 29pc.

A spokesperson for the NLWA said it made "economic sense for all the parties involved" to recycle as much as possible.

However while the NLWA made £1.35m from the recycling sold on by Biffa and Bywaters, there are no rewards or penalties for the firms to make sure recycling is recycled. "Removing contamination is a key part of ensuring recyclable material is of the highest possible quality," said a spokesperson. "We carry out regular audits and checks, including on the income received from the sale of recyclable material, to ensure that recyclable material is being dealt with responsibly by our contractors." "The majority of residents put the right things into their recycling bin, but there's a proportion that do not.

"If the recycling is too contaminated on delivery to the MRF the lorry load of recycling has to be rejected.

"Earlier this year NLWA and Biffa ran a media campaign to raise awareness of the problems caused by residents putting used nappies in the recycling. However, one of the challenges in areas like north London is the transient nature of the population and the difficulties this creates with communicating messages about recycling. We are therefore very glad that the issue of contamination is now getting this level of attention."