The sales contravened the 1998 EU Code of Conduct on Arms Exports that restricts business with countries facing internal conflicts or with poor human rights records and a history of violating international law, the Times reports.
The EU called for peace talks after the collapse of the 2002 ceasefire and stated that it did not support any military solution. However, the sales still went ahead.
"The EU had an obligation not to supply these things," Malcolm Bruce, a Liberal Democrat MP who was in Sri Lanka last month told the paper. "There were too many unanswered questions. With hindsight, Britain's sales did violate the EU code of conduct."
Slovakia is the only country to record the actual delivery of the arms, confirming the shipment of 10,000 rockets worth £1.1 million.
It is not known whether British-made weapons were used during the final, bloody, five months of the war, when the UN estimate that 20,000 civilians were killed.
"I think we need answers about what these were used for," said Mike Gapes, a Labour MP and member of the Committee on Arms Export Controls.
The US also sold millions of pounds worth of military equipment to Sri Lanka, but suspended sales in early 2008 when full details of rights abuses began to emerge.
In 2008 alone, the British Government approved the sale of £4 million of equipment, including sonar detection components and military communication equipment. "We should have been sharper off the mark and so should the EU," said John Battle, a Labour MP, former Foreign and Commonwealth Office minister and now a member of the Committee on Arms Export Controls.
MPs and MEPs called for the EU's code of conduct on arms sales to be revised to increase transparency across all countries and prevent the sale of arms that could be used to violate human rights.