The UK’s largest water and wastewater services provider is keeping tight-lipped about the firm responsible for pouring concrete into a sewer, causing major disruption to motorists.

Watling Street (A5183) in Radlett underwent an emergency closure for five days to enable Thames Water to deal with a blocked sewer near Radlett Station.

The sewer was completely obstructed by concrete, causing sewage to back up.

Watling Street was reopened on Tuesday but two-way traffic lights are in place while one final section of the work is completed by tomorrow afternoon (Friday).

Motorists in the area have been affected by diversions as a result of the road closure.

Yesterday a spokeswoman for Thames Water told the Herts Advertiser that the firm now knew who was responsible for the concrete getting into the pipes.

She added: “It was an accident and we’re dealing with them directly over costs. We’re not giving their details to the press.”

About 16 metres of a six-inch sewer pipe were affected, and had to be replaced.

Thames Water operations manager Mark Grimshaw said: “Normally blockages are caused by fat, oil and grease building up in the sewer but unfortunately in this case it was concrete, so we couldn’t just jet it through.”

Mark said the concrete had adhered to the pipe, so the pipe had to be removed.

He added: “This is very frustrating and not the first time damage has been caused by private contractors pouring concrete into our sewers.”

Mark said that while the priority was to fix the problem and restore normal service, Thames Water would reclaim costs from the firm responsible.

Cllr Terry Douris, Herts county council’s cabinet member for highways, said: “We are extremely frustrated that this situation has arisen due to what appears to be someone else’s irresponsible actions and residents affected by this disruption have our sympathy.

“Due to the nature of these works council taxpayers will not be footing the bill.”