Bitcoin has exploded over the last few weeks, adding to gains through 2020 that's seen it more than double in value.

The bitcoin price has this week shot past $18,000 per bitcoin, causing jubilation among long-suffering bitcoin supporters and causing many to recall bitcoin's late 2017 bubble—though it's happening without 2017's mainstream bitcoin mania.

Now, a leaked report from Wall Street giant Citibank has revealed a senior analyst thinks bitcoin could potentially hit a high of $318,000 by December 2021, calling it "21st century gold."

"The whole existence of bitcoin has been characterised by unthinkable rallies followed by painful corrections, the type of pattern that sustains a long term trend," Citibank's global head of CitiFX Technicals, Tom Fitzpatrick, wrote in the note to institutional clients that was leaked on Twitter—asking: "Are we on the cusp of another such structural development?"

Pointing to a chart that shows three bitcoin bull runs over the last decade, the note suggests that the current rally could "potentially peak in December 2021, at the high of the channel, suggesting a move as high as $318,000 [per bitcoin]."

"Time will tell if we end up seeing such lofty levels but the backdrop and the price action we are looking at clearly suggest the potential for a major move higher nonetheless in the next 12-24 months."

The note claims "the arguments in favour of bitcoin could well be at their most persuasive ever," referencing bitcoin's growing reputation as digital gold and "central banks increasingly discussing the digitization of currencies."

Bitcoin is also expected to benefit from "the change in monetary policy and simultaneous opening up in fiscal policy" that could devalue traditional currencies.

This chimes with a separate note released by Citibank this week that predicts the coronavirus vaccine roll out and loose monetary policy will weigh on the U.S. dollar through 2021.

"When viable, widely distributed vaccines hit the market, we believe that this will catalyze the next leg lower in the structural USD downtrend we expect," Citi wrote, according to reports. "Given this set-up, there is the potential for the dollar's losses to be front-loaded, with the USD potentially falling by as much as 20% in 2021."

Bitcoin's rally this year has been fuelled by renewed interest from Wall Street and support from a host of big-name investors, many of whom have given their backing to bitcoin as an emerging hedge against a wave of inflation that could be on the horizon.

Amid bitcoin's latest rally, the bitcoin and cryptocurrency community has cheered what has been taken as vindication of their much-maligned belief in bitcoin's eventual success.

"The dawn of the next great crypto bull run may be upon us," Paolo Ardoino, chief technology officer at British Virgin Islands-based bitcoin and cryptocurrency exchange Bitfinex, said via email. "We are seeing bitcoin prices that have not been reached in years. This time, institutional money is infusing bitcoin’s gains."