The rate-rigging scandal that has rocked the banking industry is showing no signs of abating as US authorities summoned three British banks for questioning.

Royal Bank of Scotland, HSBC and Barclays were among seven banks handed legal notices demanding that they assist in an inquiry by the attorneys general of New York and Connecticut.

The move raises fears over future penalties and further damage to the already battered reputation of Britain's banks, with several other legal cases in multiple countries looking at the manipulation of Libor.

Ian Gordon, analyst at brokers Investec, said there was "plenty more reputational damage and regulatory fines coming down the pipe" but added the issue was now largely "in the past" for Barclays.

Shares in the banks were broadly unaffected by the development, which comes after Barclays was fined £290 million by UK and US regulators for manipulating the Libor, a key interbank lending rate that affects mortgages and loans.

Barclays declined to comment, while HSBC and RBS both referred to statements made with their half-year results acknowledging ongoing investigations.

Details of the subpoenas were not given, but the legal notices are effectively requests for information backed with the force of the law.

Libor - the London interbank offered rate - is used to set the interest rates on trillions of dollars in contracts around the world, including mortgages and credit cards.

A self-policing system, Libor relies on information that global banks submit to British banking authority the British Bankers' Association.

Barclays admitted that it submitted figures that were lower than accurate for its interbank borrowing, including during the financial crisis in the fall of 2008, making Barclays appear healthier than it was.

A number of Barclays employees also submitted false rates in a bid to line their own pockets, according to the findings of the Financial Services Authority.

UBS filed a report with regulators on 31 July saying that agencies, including state attorneys general, were examining whether it and other banks had tried to manipulate the rate. Citigroup and JPMorgan Chase also declined to comment.

Last week Sir David Walker - the City grandee who oversaw a review into bank governance for former prime minister Gordon Brown - was unveiled as the new chairman of Barclays.

Sir David will join as non-executive director from 1 September before succeeding chairman Marcus Agius, who announced his intention to resign in the wake of the Libor-rigging scandal.

He said his priority was finding a new chief executive after the affair claimed the scalp of Bob Diamond.