Senior civil servants hold shares worth millions of pounds in a company being given substantial Whitehall contracts.

The officials are former employees of Accenture, the management consultancy, which gave them individual holdings worth up to £10m each.

Several kept their shares after entering government despite the possibility that such large holdings might create a conflict of interest. Accenture earns more than £100m a year from government contracts.

Inquiries by The Sunday Times have established that at least five former Accenture executives held substantial shareholdings while in government.

One is Gill Rider, director-general of personnel in the Cabinet Office, who has shares worth £4.6m in the company, which has its headquarters in Bermuda. She has already cashed in an estimated £5m in shares. Her colleague Tim Gbedemah left the Home Office last month. He has a shareholding in Accenture worth £10.3m.

The disclosures have this weekend led to fresh calls to tighten the rules governing the disclosure of civil servants’ financial interests.

Officials are required to report any potential conflict of interest to their superiors but – unlike some of their American counterparts – do not have to make a public declaration.

Sir Alistair Graham, former head of the committee on standards in public life, said: “Civil servants’ outside interests should be publicly available so people can be satisfied that the advice given to ministers by civil servants is genuinely independent and not in any way tarnished by conflicts of interest.” Accenture was formed in 2001 from the consulting arm of Arthur Andersen, the auditor that was later tarnished by its links to Enron, the Texan company that collapsed amid financial irregularities.

In recent years Accenture has profited from Labour’s generous spending on outside consultants. The Commons public accounts committee has described some of the spending as “sheer profligacy”.

Accenture’s closeness to government is reflected in the number of its executives who have moved to civil service positions. Many maintain good contacts with former colleagues.

One Accenture executive said that in the past two to three years a “massive” number of Accenture people had gone to the government. “They [the ex-employees] have to be seen to be whiter than white . . . but we do get preferential treatment.”

Unlike British companies, Accenture’s Bermuda share register is not publicly available. Our reporter had to buy shares and make repeated phone calls over six weeks before the register was handed over.

The register – dated June 2008 – shows that Rider, who is paid £180,000 to improve leadership and skills in Whitehall, owns 238,000 shares in the company. At the current share price of $39, this means her holding is worth £4.6m. Also on the register is Susie Gear, the director of change at the Cabinet Office, who has 31,000 shares in Accenture worth £608,000. She has sold about £340,000 worth.

The Cabinet Office is understood to have spent an estimated £200,000 a year on Accenture consultants. Accenture says there are no current contracts.

A Cabinet Office spokesman said its officials had declared the financial interest to the permanent secretary. “There is unlikely to be a direct conflict of interest with Cabinet Office dealings and if there were, the official would not play a role,” she said.

Gbedemah, a director in the border and immigration agency until last month, was given an OBE for public service last year. He declared his £10.3m stake of 529,000 Accenture shares to the permanent secretary of his department.

The Home Office has given a £650m “e-border” contract aimed at improving immigration controls to a consortium that includes Accenture.

Two further millionaires are senior figures in the Immigration and Passport Service (IPS).

James Hall initially retained a shareholding in Accenture when he joined IPS as its chief executive almost two years ago. He originally had at least £8m worth of shares in Accenture, according to a 2003 company register. The last of his shares were sold last year.

Last year Accenture bid for work on the national identity card scheme, a lucrative contract overseen by Hall’s department. The company was short-listed but pulled out.

Hall is a close friend of Lis Astall, managing director of Accenture’s European government business (who has shares worth £4.2m), and was a guest at her holiday home in the Balearic Islands last month. Accenture says Hall declared his visits to Astall when he first joined the government.

His colleague Bill Crothers, an IPS director, has sold his shareholding since joining the civil service in April 2007. The estimated worth of his total shareholding since 2003 would have been at least £5.9m.

A Home Office spokesman said the two men had declared their financial interests and had sold them as soon as regulatory rules in the US (where the shares are listed) had allowed them to.

A spokesman for Accenture said the company had the “highest standards of ethics which we rigorously adhere to” in winning business.

“We fully comply with the rules and regulations for government procurement,” he added.

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