Bob Crow attended a meeting of P&O's RMT members in Dover

Over 800 workers at Dover and 400 at Portsmouth are to be made redundant, the company announced in September.

Mr Crow met staff, who have not yet been told whose jobs will go, at an open meeting in Dover on Tuesday.

He said if the company made no concessions the RMT would cause as much disruption at the ports as possible.

Legal requirements

"Our first aim would be to negotiate with the employer but if it doesn't make any concessions we will have to take action to get proper job security for our members," he said.

The RMT has more than 1,000 members at Dover.

To comply with the law, it is likely that any industrial action would take place towards the end of the year.

Malcolm Dunning, RMT spokesman in Dover, said industrial action was not a foregone conclusion.

"The law sets out legal requirements that the membership will have to consider," he said.

The RMT has previously told members it will resist compulsory redundancies.

P&O said it was still negotiating with union members but that the job losses were essential to save the majority of jobs at the port.

It has said it was forced to make the cuts following increased competition from no-frills airlines and Eurotunnel.

Prime Minister Tony Blair has said in the Commons he was "sorry" about the job losses and those affected would be helped by the Department for Work and Pensions.