Some of the heaviest snowfalls for years are forecast for this week, bringing the prospect of delays and cancellations for rail and air passengers and hazardous conditions on the roads.

Forecasters issued a severe weather warning yesterday, with predictions of up to 6in (15cm) of snow in South and Mid Wales and about 2in for London by tomorrow morning.

Parts of southern England could have snowfalls of 4in. London is expected to have its heaviest snowfall since just before the new millennium, when blizzards with temperatures of minus 5C (23F) swept across the South East. The Midlands are also expected to have bad weather.

Last month a snowfall of less than an inch caused widespread disruption for travellers. Heathrow and Gatwick cancelled more than 20 flights, the Tube was badly hit and 340,000 rail commuters suffered delays and cancellations. In London, more than 20 points failed at key stations.

The Met Office said that it was expecting heavy snow to develop across southwest England and much of Wales tonight. “This is expected to extend across the Midlands, London, the South East of England and East Anglia during Thursday morning.” It added that the snow would cause disruption to travel networks.

Transport authorities are preparing for the potential travel disorder to try to avoid a repeat of last month’s problems on the roads, Tube and trains.

A spokesman for Network Rail said that the company was liaising with the Met Office. “When you have got weather like this, we can’t guarantee it won’t cause any delays. But we are doing everything we can to make sure things run as smoothly as possible,” he said.

Network Rail plans to scrape ice from tracks and spray them with a defrosting agent early tomorrow. Empty trains will then be sent out to test the conditions before scheduled services begin. Automatic heaters for the points will also be in operation.

The routes most likely to experience problems in the cold weather include lines from Kent and Sussex into London. These are prone to disruption because they have a third electric rail on the ground that can be affected by ice.

On the roads, the Highways Agency has 400 salt-spreading vehicles on standby. England’s motorways and main A-roads will be treated with salt before the snow arrives, and work to treat and clear the roads will continue throughout the cold snap. The agency has advised motorists to check weather forecasts and road conditions before they set out and to listen to travel bulletins while they are on the road.

People are also advised to make sure that they are prepared with warm clothes, food, water, boots, deicer, a torch and spade, particularly for long journeys.

Contingency plans are in place at airports. A spokesman for Heathrow said that a team would be on standby to ensure the runways were safe.

Transport for London (TfL) said that it had ordered more than 4,000 tonnes of salt, which was located at various depots.

A spokeswoman said that TfL was working closely with Metronet and Tube Lines, which are responsible for the maintenance of the Tube’s infrastructure, to ensure that disruption was kept to a minimum.

Paul Knightley, forecaster for MeteoGroup UK, the weather division of the Press Association, said that cold air from the north, mixing with an “active” mild weather system from the Atlantic, would bring the snowfalls.

“The snow will be potentially disruptive in London because it is predicted for rush hour, and is going to cause some significant disruption,” he said.