The acquisition of A&L will make Santander, which bought Abbey for £8.5 billion four years ago, the UK’s second-largest mortgage lender.

Its offer is worth 299p a share, or 317p once shareholders’ 18p interim dividends have been paid. Investors will get one Santander share for three A&L ones and a 36% premium on the deal day’s closing share price.

However, the share price has since rebounded to 328Çp. Many investors feel they are being sold short given that the shares were worth more than £12 each last year.

A&L said investors might suffer more without a deal of this kind. Its chief executive, David Bennett, said: “These are turbulent times and when we look forward it’s not clear when that turbulence ends.”

Mortgage holders, brokers and market watchers have their own ideas about what is likely to happen, though. Darren Cook, mortgage expert at the analyst Moneyfacts, said: “The merger of these two high- street names will bring further reduction in consumer choice.”

As with mortgages, A&L savings accounts are unlikely to change much in the short term. There are concerns that its straightforward range will be brought into line with Abbey accounts, which offer better rates but can come with catches.

Huge numbers of people have switched their current account to A&L in recent years thanks to its market-leading Premier Direct account, which has prompted a wave of more competitive accounts.

Michelle Slade of Moneyfacts, a financial-data firm, said: “A&L really laid down the gauntlet to other institutions with Premier Direct. If it disappears, it will be a huge loss to the current-account market.”

However, Abbey has become more aggressive in its pursuit of customers recently, launching table-topping deals such as its Zero credit card.

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