GM has been having a tough time in the US market

GM shares fell almost 6% on Wednesday to close at an 18-year low of $21.29, having already fallen 30% this month.

The car giant has suffered a slump in sales in its domestic market, declining market share and high labour costs.

The latest share fall was sparked by fears of a strike at Delphi, which is in bankruptcy protection.

Strike fear

Unions have threatened industrial action after Delphi, GM's largest supplier, said it would seek large wage cuts to improve its finances.

There are fears that any strike could disrupt production at GM, at a time when it is already facing huge problems.

If there is a continued dip in US market share, board members will have to say enough is enough Kevin Tynan, Argus Research

GM sales have fallen sharply in each of the past two months.

High gasoline prices have reduced the popularity of sports utility vehicles (SUVs).

The company suffered a $3.8bn loss in the first quarter of the year and recently said it would have to cut 25,000 jobs by 2008.

Under pressure

Industry experts said pressure was growing on the company's chief executive Rick Wagoner.

"If there is a continued dip in US market share, board members will have to say enough is enough," Kevin Tynan, group analyst at Argus Research, told Reuters.

"Even though it is not completely Wagoner's fault, you just have to shake things up."