The company had initially hoped to only have to cancel crossings up to the end of April but have now prolonged this after Guernsey's government announced it was to lengthen its lockdown restrictions.
Paul Luxon, chief executive of Condor, said that all affected passengers would now be offered a travel voucher valid for 18 months or could rebook free of charge.
'We have to react to the changing circumstances and government-imposed constraints on travel, which have regrettably led us to cease Poole and St Malo sailings for a further two weeks,' he said.
'This is a very difficult and inconvenient situation for everyone, as we have now cancelled seven weeks of our busy spring schedule which are so important for the visitor economy and travel for Islanders.'
He added that their call centre teams were now extremely busy dealing with the unprecedented situation and asked for customer's patience.
Condor's freight services continue to operate as normal.