Nasdaq has criticised the lack of co-operation between rival US exchanges before the introduction of trading rules and offered to co-ordinate testing before the next stage of implementation.

The new rules, known as Regulation NMS, introduced in March, require trading companies to use electronic trading wherever possible and achieve the best price for each trade. Exchanges have had to upgrade systems and provide greater transparency on transactions but brokers are not obliged to comply fully with the best execution requirements until July 9.