St Paul syndicate 340 and Crowe syndicate 53 were the worst performing insurers during Lloyd's last quarter.
According to Leadenhall Insurance Consultants, both syndicates' losses increased in 1999 and 2000.
Director Karen Sinden said: "Looking at the 1999 estimates, since the last quarter the estimate for all syndicates has increased by 2.3% to an overall estimated loss for the eleventh quarter of 16.7% of total market capacity.
"This would produce a loss in the region of £1.6bn for the 1999 year of account."
Sinden said the majority of the syndicates, such as HIH Cotesworth 535 and 1688, had ceased underwriting in the last quarter.
But she added: "Investigation into the risks underwritten on these syndicates indicate a worse loss than originally envisaged".
In relation to the anticipated 2000 results, Sinden said the overall deterioration for Lloyd's has worsened by nearly 8%.
She said: "The estimated result after the seventh quarter now stands at 14.6%. This is much worse than at the same point in 1999 and would give a loss of approximately £1.5bn."
Syndicates 340 (St Paul), 53 (Crowe), 271 (Kingsmead), 861 (XL Brockbank), 566 (Limit), and 1069 (Cotesworth) bore the brunt of the World Trade Centre tragedy. The syndicates have a combined net loss of approximately £250m.
Sinden said as Lloyd's total net loss for World Trade Centre for 2000 is estimated at £394m, these syndicates alone have approximately 63% of the net loss between them.