A US court claim for $13billion (£8.3billion) has knocked Royal Bank of Scotland shares and put the spotlight on the government's plan for an early start to selling down its 78per cent stake.

The bank is said, in documents filed in a US court, to have misled investors in the sale of about $32billion (£20bn) of mortgage-backed debt before the crash.

RBS has so far set aside only £1.9bn to deal with the claims. Analysts have said they expect the bank to have to pay anywhere between £3bn and £9bn to settle the matter. RBS declined to comment.

The size of the potential settlement could make it more difficult for UK Financial Investments to accelerate its sell-off plan in the coming months, industry sources said. Outgoing RBS chairman Sir Philip Hampton and Conservative MP Mark Garnier have both suggested that the issue needs to be resolved before the government can offload its shares.

The Federal Housing Finance Agency brought the case in 2011 and it is yet to go to trial, although RBS has made repeated references to the legal proceedings in disclosures to shareholders.

The settlement claim was disclosed in a filing to a New York court, but the total also covers a second similar claim in a Connecticut court.

It is based on a previous judgement in a separate case, which last month saw Nomura and RBS ordered last month to pay a collective $806 (£513m) for making false statements in selling mortgage-backed securities to US housing agencies Fannie Mae and Freddie Mac.

RBS hopes to return to profit in 2016 for the first time since its £46bn bailout in 2008. Its £3.5bn loss last year was due almost entirely to past misconduct costs, including £400m for PPI compensation, £430m for US fines related to foreign exchange rigging, and another £444m of conduct and litigation charges, in the fourth quarter.