Spotify could be valued at $20 billion when it goes public and eventually be worth $100 billion, according to one investment bank.

Spotify announced in July that it had hit 60 million paying users. GP Bullhound sees this hitting 100 million by mid-2018, quicker than it had originally thought. And by 2020, the user base could hit 500 million, with 200 million users being premium subscribers.

This will be achieved by penetrating new markets, but along with that will come fewer paying subscribers as a percentage of total users, and perhaps lower average revenue per customer, due to a dilution effect of cheaper subscription plans.

"Spotify have introduced family plans and student discounts and if we factor in that emerging markets have a much lower average revenue per premium subscriber we believe that Spotify will see a steady decline in revenue per premium subscriber moving towards 2020 compared to today's value," GP Bullhound said in its report.

"This is the reason why we have decreased our estimated average revenue per premium subscriber to $80 — compared to $88 in 2015 and $89 in 2016."

But the investment bank said that advertising could lead to a higher revenue per non-premium subscriber than previously anticipated.

Even though Spotify is set to grow quickly, it is likely to continue recording an operating loss which could "distract investors from the true value that is being created," GP Bullhound said.

But the music streaming service's return on investment should improve. Currently for every dollar it invests in a premium subscriber, it gets $3 back. The return could increase to $5 by 2020 as revenue per user stabilizes, churn decreases, and gross margin improves.

Gross margins will improve as Spotify strikes better deals with music labels thanks to its rising user base.

GP Bullhound's forecast of a valuation for Spotify of $20 billion after its initial public offering (IPO), is based on it having 70 million premium subscribers and 100 million non-premium users. Given that this will likely be reached in the next few months, the investment bank said its valuation thesis is "conservative".

Eventually, the Swedish company could hit a $100 billion valuation.

"Given that Spotify's growth continues and that emerging market growth keeps average revenue per premium subscriber relatively in shape we envision that Spotify has a long term potential of being valued at $100 billion," GP Bullhound said.

It's important to note that Spotify is going to carry out a traditional IPO. Sources told CNBC earlier this year that Spotify is going to go public in a so-called direct listing.

During an IPO, the company offers investors shares before it goes public, which allows underwriters to set an initial share price. A direct listing only allows investors to buy shares through the open market, and no price is predetermined.