Hundreds of employees of the digital bank Monzo are being given the opportunity to sell part of their stakes in the company as its valuation soars to £4.5bn.

Sky News has learnt that Monzo notified staff on Thursday that it was launching a secondary share sale backed by a number of the world’s leading technology investors.

Sources close to the deal said employees were likely to sell tens of millions of pounds-worth of stock as part of the deal, which is being launched less than three weeks before Rachel Reeves, the chancellor, is expected to increase the rate of capital gains tax in her inaugural budget.

Monzo, which has more than 10 million customers, has become one of Britain’s most successful, and valuable, fintech companies.

It employs more than 3,700 people.

Earlier this year, it raised more than £500m by selling newly issued shares to a group of investors led by Capital G, a division of Alphabet-owned Google.

That primary share sale valued the business at £4.1bn.

The latest share sale is understood to involve existing Monzo investors StepStone Group and GIC, the Singaporean sovereign wealth fund, buying stock from employees.

One insider said it would set a new floor for Monzo’s valuation as it progresses towards an initial public offering at some stage in the next couple of years.

Founded in 2015, it is now profitable and has diversified into investments and instant access savings accounts.

In recent months it has also launched pensions products and accounts aimed at under-16s.

Monzo now ranks as the seventh-biggest bank in Britain by number of customers, with one in five adults and one in 16 businesses now banking with it.

The company is among a new generation of banks which have emerged since the last financial crisis and begun to accumulate a significant share of the UK retail banking market.

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Rivals include Starling Bank and Revolut, which was recently valued at $45bn and has just been awarded a banking licence by British regulators.

Monzo has recovered spectacularly from a difficult period when it emerged that the City watchdog was investigating it for potential breaches of anti-money laundering and financial crime rules.

It has revamped its corporate structure as it pursues an international expansion strategy that will serve as the prelude to a stock market listing.

Monzo Bank Holding Group was established to avoid the company facing punitive capital treatment by British regulators as it launches in new overseas markets.

Other Monzo investors include the Chinese group Tencent, Passion Capital, Accel, General Catalyst and Hedosophia.

Monzo is run by TS Anil, its chief executive, and chaired by Gary Hoffman, one of Britain’s most prominent bank executives.

A Monzo spokesperson declined to comment.