Built for a city half London’s current size, however, the system is now overflowing. As often as once a week, raw sewage is forced into the Thames, a sharp change from the 19th century, when the newly built system overflowed less than once a year.

The increasing flow of raw sewage — the result of the loss of green spaces to absorb rainwater as much as population gain — violates European environmental law, the European Commission said in 2009, and the government has promised to act.

But in an era of austerity and strained budgets, it is not the government that is paying the $6.6 billion bill but Thames Water, a private company with shareholders. The government is to underwrite the risk, which means that it will act as the financier of last resort in the case of major problems during the construction, but it will otherwise not pay for the new system.

There is a catch, of course: it is actually the customers of Thames Water who are paying for the project with higher water bills, a prospect almost as horrendous to today’s Londoners as the river’s stench was to their 19th-century forebears. Water bills for Thames Water’s 14 million customers in and around London are to rise to as much as $700 annually from $570 for the foreseeable future, the company said. The money will be used to repay the initial investors, who will also own the new system. And even then, the tunnel will remain the property of the company, a prospect that further rankles.

A variety of local politicians and industry experts say the plan is akin to pouring money down the drain, that construction estimates are far too low, and that there are cheaper and less disruptive alternatives. “The costs might end up at 10 billion pounds,” or about $16.16 billion, said Nicholas Botterill, council leader of Hammersmith and Fulham, districts in southwest London, “and I don’t want the country to waste that much money.”