The head of Royal Bank of Scotland's (RBS) investment banking arm is to step down - forfeiting £4 million in shares.

John Hourican will leave the bank at the end of the month and receive a parting gift of a year's salary in lieu of notice, worth around £700,000, according to Sky News on Tuesday.

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RBS is believed to be facing a £500m fine over the Libor rate-rigging scandal

He was asked by the bank's board to forfeit the £4 million he is owed in shares, the news channel reported.

Mr Hourican was widely expected to shoulder the blame for RBS's alleged role in the interbank rate-rigging affair, although he is not believed to be directly implicated.

He has headed up RBS's wholesale bank since the group's bail-out at the height of the financial crisis and has already overseen a mammoth restructuring, with the division's workforce slashed by around 10,000.

It is understood the taxpayer-backed lender plans to announce a shake-up that will see its markets business split from its international banking division.

It is thought that Mr Hourican will not be replaced when the investment banking arm - which employs around 16,000 people - is broken up, with heads of the two new divisions instead reporting directly to chief executive Stephen Hester.

RBS will reportedly face criminal charges and a £500m fine for its role in the Libor rate-rigging scandal this week.

It is likely to announce the settlement with the Financial Services Authority and American regulators tomorrow.

RBS - which is 81% state owned - is thought to be under pressure from the government to pay the fine with cash from its bonus pot to ensure taxpayers do not suffer, but the report suggests traders are still set to receive bonuses worth hundreds of millions of pounds for 2012.

The lender is one of about 20 banks which are being investigated over involvement in manipulating the rate, which governs the price of more than 500 trillion US dollars-worth of loans and transactions around the world, including household mortgages.

RBS's fine is set to dwarf the £290m settlement agreed by Barclays last year over its involvement.

American prosecutors, who have already charged two former employees of Swiss bank UBS over the scandal, are said to be keen to press criminal charges at RBS.

UBS has already agreed a near £1 billion settlement with regulators.

An RBS spokeswoman declined to comment.