The manager of Norway's $1 trillion sovereign wealth fund should have more freedom to invest in unlisted equities - a proposal that could help the fund better capture the growth of US tech firms.

The world's largest sovereign wealth fund, which pools Norway's revenues from oil and gas production in stocks, bonds and real estate abroad, is currently only allowed to invest in unlisted equities if a listing is imminent.

The fund's management has complained in the past about being unable to invest in companies early on in their growth, particularly US tech firms, as companies have tended to seek a listing late on in their growth process.

"The (central) Bank is of the opinion that the wording of the current regulation that the board must have expressed 'an intention' to seek a listing should be amended," the central bank said in a letter to the finance ministry published on its website on Wednesday.

"To date, we have interpreted this mandate regulation as requiring a concrete board resolution to apply for listing in the near future.

"Our experience is that such resolutions are passed relatively late in the process, and that a number of transactions that might have been relevant to the fund have not therefore been pursued."

The letter then suggests that a limit of 1% of the equity portfolio could be dedicated to that type of investment, which would have corresponded to 63.5 billion crowns ($7.1 billion) at the end of the second quarter, according to a Reuters calculation.

Investing in unlisted equities has been a vexed issue for the fund. In 2014 it joined US investors BlackRock and Waddell & Reed to buy a $1.6 billion stake in motor racing's Formula Oneas an IPO was planned.

After the investment was made, Formula One cancelled plans for the IPO, putting the fund in contravention of its own mandate. The fund came under political and media criticism fo rthe investment and the fund's CEO acknowledged mistakes had been made.