For households plagued by hosepipe bans and rising bills, the prospect of an £8 billion payday for the owner of Thames Water will be hard to swallow.

It has been a year of negative headlines and industry fines - particularly for Thames Water - and yet investors are falling over themselves for a slice of the sector.

Last week, Anglian Water owner AWG recommended an offer valued at £2.25 billion - double its value three years ago - and other firms appear in play.

The latest speculation surrounds a privaty equity bid for Severn Trent, which could value the Birmingham-based utility at £5 billion.

The attraction of regulated water companies may not be obvious to customers, but they are the City's prized asset, worthy of bids from investors worldwide.

Thames Water, which is owned by German utility RWE, is due to be sold in the next few days, with contenders reported to include Australian banking group Macquarie and Goldman Sachs.

Ten regional English and Welsh water companies emerged at privatisation in 1989, but investment bank UBS has pondered whether there would be any quoted companies left for stock market consumption.

In the space of a year, East Surrey, Bristol Water and, most recently, South East Water have been taken out.

As the high-profile sales of BAA and P&O proved, infrastructure assets are prime targets because of their stable, transparent cash flows.

In an example of financial engineering, owners usually load the business with debt and bank on the guaranteed returns offered by the water regulator. Growth rates will be higher than the cost of servicing the debt.

Around $160 billion (£86.2bn) is thought to have been raised by private equity companies for acquisitions across a range of sectors this year.

At least eight parties are believed to have expressed an interest in either Anglian Water owner AWG or Thames Water.

Given that only two of the eight will be successful, it appears certain there will be further private equity bids in

the sector. But there are concerns that the race for control of such assets has overheated valuations, adding to speculation that the bubble is about to burst.

For Severn Trent - one of the largest of the remaining listed water companies - it already trades at a 22 per cent premium to its regulated assets, limiting any potential upside as recent takeover offers have normally carried a 25 per cent premium.

Many industry experts are surprised at the frenzy, mainly because the current five-year regulatory period - which allows price rises in

return for investment in the network - runs out in 2010 and it is quite possible that Ofwat could look at the current bid activity and lower allowed returns.

Morgan Stanley said: "UK water companies have been high profile in the media in recent times, with accusations of earning excess returns.

"Such events could have a bearing on the regulator as we move towards the negotiation stages of the next regulatory review.

"It should be remembered that Ofwat's primary duty is to the customer."