Elon Musk's tumultuous plan to buy Twitter is on the verge of collapse — but the world's richest man could still find himself forced to pay up.

Mr Musk has now publicly mocked the social media giant, after his attempt to bail out of the $US44 billion ($61 billion) bid was revealed earlier this month. 

But as Twitter executives vow to take him to court to honour the deal, both sides are staring down the barrel of a costly legal battle.

Here's what we know so far.

How much money could Musk be forced to pay for Twitter? 

The Tesla CEO's original bid was for $US44 billion.

Originally, the social media company rejected Mr Musk's advances.

But now Twitter has announced it plans to sue him in an attempt to force him to pay up and buy the company. 

Even if he's not forced to pay the full original amount, Mr Musk could still be made to pay a $US1 billion break-up fee if he can't prove Twitter breached their agreement. 

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However some legal experts believe both sides will instead reach a settlement, both to avoid Mr Musk being forced to buy a company he doesn't want and to stop a legal battle decimating Twitter's morale and share prices. 

Why did Elon Musk pull out of the deal in the first place? 

He claims Twitter breached their agreement by failing to provide enough information on spam accounts, that Twitter misrepresented the number of spam accounts on the site, and that it failed to consult Mr Musk when firing senior employees recently. 

Mr Musk's lawyers wrote to Twitter's chief legal officer saying Mr Musk believed the company was "actively resisting and thwarting" his rights to access data and information. 

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“Twitter’s latest offer to simply provide additional details regarding the company’s own testing methodologies, whether through written materials or verbal explanations, is tantamount to refusing Mr Musk’s data requests,” the letter read. 

It also warned Twitter Mr Musk reserved his right to "terminate". 

How did this whole saga begin? 

Mr Musk had been hinting at buying Twitter since late March. 

Twitter revealed then that he had contacted members of its board and told them he was interested in either joining the board, buying the company or starting up his own social media platform. 

On April 4 he became the company's largest shareholder.

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Twitter at first offered him a seat on the board, then announced less than a week later he would not be joining. 

But shortly after that the two sides agreed that Mr Musk would buy Twitter for $US54.20 per share — "420" being a marijuana reference.  

Inside the company, the announcement was met with confusion and falling morale among staff. 

As Twitter executives prepared to move forward, the company launched a hiring freeze, halted spending and fired two top managers. 

What happens next? 

The best outcome for Twitter may be just to accept a lower offer from Mr Musk. 

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As all this plays out Twitter shares continue to fall and Tesla shares climb higher. 

If Twitter does sue, the case could be fast-tracked to the Delaware Court of Chancery, a prime venue for US corporate battles. 

After that the case can be appealed to Delaware's Supreme Court, whose decision is final.