Shares in Aer Lingus will not officially list on the Irish and London stock exchanges until next Monday. But to all intents and purposes, trading began yesterday, with prices being recorded by the exchanges' electronic trading systems.

Any trading ahead of next Monday is conditional as in theory the initial public offering (IPO) or flotation could be pulled from the market by its sponsors at any stage up that point.

If this happened, the dealings would be voided and any losses would be the responsibility of the parties undertaking the deals.

Investors who bought shares in the flotation will not have them credited to their Crest accounts - individual electronic accounts managed on their behalf by brokers - until next Monday. The official listing is unlikely to have any material impact on the price level that will emerge over the next few days of conditional dealings

"The price that has been set for the so-called grey market will be the same one that's carried over to the exchange next week," said one source.

However, a number of institutions will start to make a market in Aer Lingus shares from Monday onwards.

Currently only Goodbody Stockbrokers, who are the joint bookrunners for the issue - are making a market.

Retail investors who hold shares bought in the IPO until 6pm on October 1st, 2007 will qualify for the bonus share incentive, which will be one share for every 20 held at that point.