[12] In 1999, the company merged with Garban to create Garban-Intercapital, the largest inter-dealer broker in the world, with more than 5,000 employees across 63 offices.

[15] In September 2013, ICAP was implicated in the global Libor interest rate scandal and fined $87m (£54m) by the US Commodity Futures Trading Commission and Britain's Financial Conduct Authority.

They were among a global group of finance professionals to be investigated, including staff from Barclays, UBS, Citigroup and RP Martin.

In January 2016 the three ICAP employees were unanimously acquitted in the UK; the US Department of Justice dropped charges against the trio in July 2016.

[21][22] By the time the deal was completed in November 2018, Spencer's stake had grown to over £700m, half of which he received in cash, raising his net worth to over £1bn.

City Index was later sold for $118m to financial services company GAIN Capital, netting Spencer more than $80m.

[24] He is Chairman, director and majority shareholder in IPGL, a private holding company making investments on behalf of Spencer and other family trusts.

He is a former investor in Temple Grange Partners, a consultancy that finds compliance specialists for financial markets.

[32] He also holds stakes in payment platform Klarna, wealth manager AJ Bell and online investment advisor Nutmeg.

[35] In January 2022, Spencer invested £10 million in a new global fund launched by Toscafund Asset Management.

[40] In April 2024 Spencer received a £210m windfall following the sale of his stake in Singapore Life, an online savings platform for Singaporean and Asian markets.

[38] Spencer's House of Lords register of interests shows stakes held in British bookmaker The Tote, Apiary Consulting, DDCAP, Incubex, Sunsave and InsureWave.

[50] He also owns 7% of Pantheon Resources, an Aim listed oil company with extensive interests on the North Slope of Alaska.

[55][56] In 2020, he became Chairman of the Centre for Policy Studies, the think tank and pressure group founded by Margaret Thatcher.

[58] Spencer is a long-standing donor to the Conservative Party, giving £250,000 during the 2024 UK General Election campaign.

[10] He opposes MiFID II, tax surcharges on UK bank profits[64][65] and stamp duty on share trading.

[10][76][77] Various public individuals have manned the phones for Charity Day, including Prince William and Kate Middleton,[78] Camilla, Duchess of Cornwall,[79] Mo Farah,[80] Chris Hoy,[81] George W. Bush, Samantha Cameron,[82] Cheryl Cole,[83] Tom Hardy,[84] Halle Berry, Daniel Craig,[85] and Chris Hemsworth.

It created the Borana Conservancy, a 94,000 acre sanctuary that in 2013 reintroduced 21 black rhinos after a 50 year absence from the region.

The Foundation also donated £100,000 to the Remember Me Covid memorial at St Paul's and a six-figure sum to the Mail Force campaign for PPE for hospitals, care homes and charities.

[92] In June 2016, Spencer married Sarah, Marchioness of Milford Haven in London, the daughter of George Alfred Walker and Jean Maureen (née Hatton), and former wife of George Mountbatten, 4th Marquess of Milford Haven – a second cousin of King Charles III.

[54] He collects twentieth century and contemporary art, including the work of Pablo Picasso, Lucian Freud and Jack Vettriano.