City staff who spent the Christmas period racing to prepare for a major piece of EU legislation that goes live on Wednesday have been warned that meeting the deadline does not mean the work is over.

Staff at firms including TP ICAP were barred from taking holiday during the festive period so that they could prepare for the sweeping markets reform, known as Mifid II, by the January 3 deadline.

But EY has warned that even though it expects large asset managers to be "substantially compliant" today the work is far from over due to remaining uncertainties, the potential for further guidance and possible inaccuracies.

"Mifid II has of course been a huge body of work for businesses, and it’s not over yet," said EY's Mifid II expert Uner Nabi. "Firms may be keen to refocus resource on other priorities as soon as possible, but work on Mifid II will have to continue with adequate resourcing for around six months into 2018."

Compliance teams across the City have been scrabbling to prepare for the new rules in time for the long-awaited deadline, which kicks-starts the biggest market reform in over a decade and heeds the lessons learnt from the financial crisis with the aim of increasing transparency.

Pegged as the biggest shake-up in the City since Margaret Thatcher’s “Big Bang” programme of deregulation in 1986, the upheaval influences almost every part of the process for those involved in buying and selling shares - from changing how analysts' research is paid for to demanding more detailed information about trades.