Barclays has delayed making its formal offer for ABN Amro by about three weeks as the lengthy regulatory process surrounding the biggest ever bank takeover weighs on its timing.

The delay, granted by the Dutch regulator AFM, means the UK's third-largest bank now looks virtually certain to wait until a crunch ruling by the Supreme Court in Amsterdam before launching its €65 billion bid for the Dutch bank.

Barclays had originally given itself six weeks from the time of its agreement with ABN Amro to launch a firm offer. That deadline was due to expire later this week.

It is understood that Barclays is keen to find out whether a shareholder vote is required on the pre-agreed sale of ABN's American division LaSalle to Bank of America. A ruling by the Supreme Court is expected in mid-July.

Having initially risen, shares in Barclays lost 4.5p to 691.5p as it promised that a formal offer for ABN is now on the cards for July 23 at the latest. Barclays plans to pay for ABN entirely in shares.

"Good progress continues to be made in relation to the pre-conditions, documentation and regulatory change of control approvals. Since the regulatory review processes relating to the documentation are not yet completed, the AFM has agreed an extension," Barclays said.

Barclays, which faces opposition to its agreed deal with ABN from a consortium led by Royal Bank of Scotland, insisted that its bid timetable would not be affected. It plans to hold a shareholder vote on its takeover proposal in August and hopes to complete the deal by the end of the fourth quarter.

Barclays was boosted last week by an advisory ruling by Professor Levinus Timmerman, a Dutch advocate-general who recommended to the Supreme Court that a shareholder vote on the sale of LaSalle was not necessary.

The Court follows advocate-generals' advice in three out of four instances.

Barclays' merger plans for ABN are conditional on the sale of LaSalle to BofA going ahead. The RBS consortium's deal is conditional on it being blocked.

RBS, which last week conceded it was disappointed with the ruling, is thought to be prepared to press ahead with a bid for the rest of ABN Amro. Joined by consortium partners Fortis and Santander, it is preparing a bid the majority of which is cash.

RBS's offer is worth about €71 billion.

RBS shares fell 16.5p to 616.5p.

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