Two US class-action suits and heightened union opposition hit Marconi this morning following an announcement of 4,000 job cuts last week.
US law firms Berger & Montague, of Philadelphia, and Alfred G Yates Jr, of Pittsburgh, accused the company in a statement of violating two sections of the Securities Exchange Act.
It is claimed the company made materially false and misleading statements about its growth prospects before the profit warnings, which wiped more than 50 per cent off its shares.
Berger & Montague alleged the company and two directors falsely assured the investing community that Marconi did not need to change its earning guidance.
Marconi said it had no comment.
Consumer products giant Procter & Gamble last month agreed to pay $49 million to settle a class-action action related to a profits warning.