They say the trade and industry secretary is making crucial decisions affecting top-name companies that are major sponsors of the dome.

Peter Mandelson is sole shareholder in the New Millennium Experience Company

The opposition MPs argue that his interest in the dome may affect his decisions on their commercial activities.

In a Commons Early Day Motion, shadow trade and industry spokesman Christopher Chope said BA, BSkyB and Tesco were committed to sponsoring the Dome for £36m. Car giant Ford is also considering putting in another £12m.

Mr Mandelson's department is involved in decisions on BA's proposed alliance with American Airlines, on BSkyB's plan to buy Manchester United football club and the Office of Fair Trading's probes into supermarket and car pricing, the motion points out.

Sole shareholder

Mr Chope points out that the minister is obliged to act "fairly and impartially and to be seen to do so" and calls on Mr Mandelson to "explain why he concluded that he will be seen to be acting fairly and impartially in these cases".

He told BBC News Online: "He's the 100% shareholder and he's accountable to parliament for the success or failure of the £100m sponsorship of the dome.

"If he doesn't obtain it, he will suffer politically."

He said that of £100m sponsorship so far obtained, some £48m was from companies that were at the centre of competition inquiries.

The key point was that the minister must be seen to be fair and impartial - and many people in industry agreed, said Mr Chope.

Conflict of interest "has not yet arisen", the DTI says

He said the companies involved may appear to be compromised by Mr Mandelson's refusal to separate his role as a regulator and as sole shareholder of the New Millennium Experience Company, which is in charge of the dome.

The minister should give up his role in the company or abandon any regulation of any industry involved in dome sponsorship, the motion says.

Legal advice

The EDM, which is a means allowing MPs to formally protest at an issue, has been signed by other Conservatives: Graham Brady, John Bercow, David Prior, Nick Hawkins and Owen Paterson.

A DTI spokesman said Mr Mandelson had pledged to stand aside from any competition issues if he felt he could not act, and be seen to act, fairly and impartially.

But no such conflict had yet arisen, the spokesman said.

Mr Mandelson has no role in attracting sponsors and had taken legal advice on whether there could be any conflict of interest.

"If that advice said an issue was shaky, he would stand aside from that issue," said the spokesman.