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London mayor Boris Johnson has raised the idea of moving water supplies from Wales and Scotland to areas of shortage in the south and east of England.

He says the obvious solution is to use the rain from the mountains to tackle water shortages in drier UK areas.

He has been in touch with Prof Roger Falconer of Cardiff University, and both believe a possible solution could be to move water via rivers and canals.

Prof Falconer said a growing population would put pressure on supplies.

Mr Johnson wrote: "Since Scotland and Wales are on the whole higher up than England, it is surely time to do the obvious - use the principle of gravity to bring surplus rain from the mountains to irrigate and refresh the breadbasket of the country in the south and east."

He added: "He [Prof Falconer] has been looking at all sorts of proposals for improving our current network of canals so as to integrate them into the water supply.

"He talks of linking up Welsh mountain reservoirs via the Wye and the Severn with the Thames, or of sending the water from the Severn and the Trent round to East Anglia - for many years the driest part of the country."

Mr Johnson said the water companies mounted all sorts of objections, such as warning that it would be a mistake to mix up water from Wales with the Thames, but added that the objections sounded "pretty much like tripe to me".

Prof Falconer, of Cardiff University's school of engineering, said he had been contacted by Mr Johnson and spoke with him for about an hour.

"My wife took a call and said 'Boris Johnson is on the phone for you'," he said.

"He had done some research on this and asked if I could comment further."

Prof Falconer said he was not aware of any proposals currently for moving water from Wales via rivers and canals but there were supply problems ahead.

"We spend all our time talking about climate change and rightly so too, but we have another big problem on the horizon and that's population growth," he said.

"The world population is going to go from six to nine billion, and the problem is that people think this is a problem that's only going to occur in the developing countries but we're going to face that problem in the UK.

"We're going to go from [a population of] 60m to 70m over the next 10 to 20 years."

He said there were a number of ways to address the issue, including moving water from Wales.

"One of the options that has been raised in the past is to raise our dams, release more water in the summer months into our rivers like the Wye and the Severn, and pipe it to the Trent, for example, then down through the Birmingham canals down to the Thames, releasing it into the upper part of the Thames down into London," he said.

Welsh Water said setting up a national water grid to move water around the UK was "a government issue and it would need to meet the ecological challenges that could be encountered in mixing water from different areas which have wide variances in quality".

that would allow water companies to "trade" water from one region to another using the water industry's network of pipes.