The move to strengthen the board with three experienced banking figures was seen as an attempt to draw a line under a stormy year in which its corporate governance under McKillop and chief executive Sir Fred Goodwin has been criticised.

One of the new directors is Stephen Hester, chief executive of real estate giant British Land, who also has significant banking experience.

Hester is credited by the City with helping turn round a floundering Abbey National and is viewed by some as the likely successor to McKillop.

The other new non-executives are Scots-born John McFarlane, former chief executive of Australia and New Zealand Banking Group, and Arthur "Art" Ryan, who retired as chief executive of Prudential Financial in the United States in 2007.

One City analyst said: "Hester is impressive and would certainly stand up to Goodwin if he chose to. For this reason he would also be a sensible eventual choice as successor to McKillop.

"The pressure on Tom has not gone away because he is seen as having largely just gone along with whatever policy Goodwin has decided."

McKillop was re-elected as chairman for three years at this year's RBS AGM.

However, some believe the fact that Hester is not a Scot would be a major hurdle for him to become the first-ever non-Scottish chairman of the bank.

They say McFarlane may emerge as the front-runner to succeed McKillop . Legal & General, RBS's biggest investor with about 5 per cent, declined to comment on the appointments.

McKillop said: "I have no doubt that Stephen, John and Art will add tremendous value to the board of RBS."

One analyst said: "It is fair to call them heavyweight appointments as they all have significant banking experience. Hester was also at Credit Suisse First Boston for nearly two decades."

But Mamoun Tazi at MF Global said: "Even if it is a step in the right direction, it is not enough. We need to see more far-reaching changes.

"While bringing in independent directors is good from a corporate governance point of view, materially what will it do to the shares? Very little." RBS's shares closed yesterday up 4p at 221.75p compared with a 52-week high of over five pounds.

RBS's board and its management have come under fire during a difficult 12 months for Britain's second-largest bank.

This saw RBS persevere with the bumper takeover of Dutch bank ABN Amro at a top-dollar price even after the banking liquidity crisis flared last August.

That was followed just months later by a perceived U-turn as the Royal announced a record £12 billion rights issue to shore up its balance sheet, and massive writedowns.