Major roadworks have been taking place on the A465 for years

The 6.2km (3.8 mile) stretch between Tredegar and Dowlais was the first section to be completed.

It is hoped the investment will help regenerate the area.

The 40km (24.8 mile) section from Abergavenny to Hirwaun was branded an accident blackspot when it was a three-lane road.

Local politicians hoped the dual carriageway would help link south and west Wales to the Midlands and northern England.

The A465 could become a lifeline linking the west and east of the valleys together Councillor Thomas

Ray Thomas, a councillor for Dowlais, in Merthyr Tydfil, said the road improvements were welcome, but "well overdue".

"We are delighted the first phase is in place, and are looking forward to the next three or four phases," he said.

He said he had been involved in a campaign to improve the road for more than 20 years, amid fears about its safety.

"It has been a long fight. The M4 corridor is, at times, well congested, and this would be an alternative route into Wales.

"The A465 could become a lifeline linking the west and east of the valleys together, and to the M50 and M5."

'Attractive location'

The section between Tredegar and Dowlais Top, which has cost around �29m, was opened by assembly Economic Development Minister Andrew Davies on Thursday.

A Welsh Assembly Government spokesman said the road was an important step in its aim "to strengthen the Heads of the Valleys corridor as an attractive location for development and to reduce the economic imbalance of the area.

"Providing a high standard road between Abergavenny and Hirwaun - a distance of some 40km - is an important component of that aim."

The contract for the design and build of the Tredegar to Dowlais Top section of the A465 was awarded by the assembly in January 2002 to Hochtief Griffiths Joint Venture.