AN ESTIMATED backlog of around four million letters is being stored in a warehouse in Dartford, according to the Communications Workers Union.

The union says the outhouse in Sandpit Road is like “a huge hangar” and opened three weeks ago.

A backlog of post, due to be delivered to addresses in London, has been building up as a result of postal strikes and lack of room in sorting offices around the capital.

A spokesman for the union said: “We have been told about an outhouse that has been set up in Dartford which will probably do London work and others.

“From our information you could fit several aircraft in there.

“It’s a huge hangar.

“It looks like it is manned by casual staff only and this was a site used previously during disputes and at Christmas.

“The last time it was used the minimum charge was £500,000 so it is likely this will operate right up to Christmas now to make it cost effective.”

The union says around 100 agency staff are being employed per shift at the minimum wage of £5.80 an hour to sort post.

It also claims staff are not trained and not put through proper security checks, which Royal Mail denies.

Clandestine footage obtained by the union found that around 300 trolleys known as yorks are being used to move mail.

A Royal Mail spokesman said: “It’s true we have a couple of temporary houses.

“Staff are fully trained and its absolute nonsense to say we are not security vetting anybody.

He added: “The Communications Workers Union has done its utmost through strike action in East London to delay the mail and Royal Mail again calls on the union to stop hurting customers and provide the service our customers need and expect.

“Royal Mail is doing everything we can including drafting in management volunteers as well as fully trained and vetted agency staff and drivers, who we sometimes call on during busy periods anyway, to keep mail moving and call upon CWU to call off these damaging strikes which only inconvenience customers and harm our business.”