NatWest was on Monday fined nearly £265 million ($350 million, 310 million euros) for breaching Britain's laws on money laundering, the financial regulator said.

A judge imposed the penalty after the bank admitted offences relating to a customer's deposit of £365 million, some of which was brought in cash to a branch in bin bags.

The bank, part of the Royal Bank of Scotland group, pleaded guilty to three offences under the Money Laundering Regulations 2007 in October.

The offences were committed between November 2012 and June 2016.

Imposing the fine at a court hearing in south London, judge Sara Cockerill also ordered NatWest to pay $4.2 million in costs and made a £460,000 confiscation order.

"Throughout the indictment period, it is accepted NatWest sought to discharge its obligations under the regulations and that it failed to do so," she said.

"It is not suggested there has been any deliberate flouting of the rules or any criminal intent."

The case was the first criminal prosecution to be brought by the Financial Conduct Authority (FCA) regulator against a financial institution under anti-money laundering laws.

Judge Cockerill said NatWest was "in no way complicit in the money laundering which took place" but said it could not have been laundered "without the bank's failings".

The court was told the firm in question -- a jeweller's in Bradford, northern England -- had been shut down by police in 2016.

Its predicted annual turnover when it was first taken on as a NatWest client in 2011 was £15 million but some £365 million was deposited over five years.

Of that, £264 million was in cash. 

Prosecutor Clare Montgomery said up to £1.8 million a day was deposited from November 2013, at some 50 NatWest branches across the country.

One branch in Southall, west London, received £42 million in cash in a 15-month period to March 2016 but it was not reported as suspicious.

About £700,000 was paid in to another branch in Walsall, near Birmingham, central England, on one day, and the cash was brought through a shopping centre in bin bags.

The amount of cash deposited was more than some branches could deal with, Montgomery said.

NatWest's legal team said it accepted its failure to enforce the regulations but maintained that the suspicious deposits had been identified and scrutinised.

"The quality or adequacy of that scrutiny is another matter," lawyer John Kelsey-Fry told the court.

NatWest chief executive Alison Rose has voiced the bank's "deep regret" at failing to properly monitor the client and pledged "significant resources" to fight financial crime.