Work to replace the 40-year-old supports on the central reservation at junction 19 started on Monday and will last nine weeks.

"These bridges have been exposed to years of salt-laden water leaking on to the columns below and general wear and tear and are in need of remedial work," said Paul Hupton, Highways Agency project manager.

"We decided that complete replacement of these supporting columns would be the quickest option as well as ensuring value for money."

Each winter the motorway and the A556, which passes over the M6 at junction 19, are gritted to make them safer for the 100,000 drivers which use them.

But as the rain washes the salt away, it sticks to the supports and eats into the concrete.

"Small amounts of concrete begin to crumble and slip away," said a spokesman for the Highways Agency.

"They fall on to the motorway which is obviously a danger to motorists."

The existing columns will be replaced by larger and more-advanced single piers which use better quality concrete to keep salt water out.

Temporary supports have been placed beneath the two bridges while work takes place and both outside lanes of the M6 have been closed.

The hard shoulders are being used to maintain three lanes of traffic but a 50mph speed limit has been imposed.