LOS ANGELES - Star trader Bill Gross ended his 43-year career with Pacific Investment Management Co (Pimco) with a barely legible, handwritten resignation note left "in the middle of the night", according to a court filing on Monday (April 4).

Pimco, stepping up its war with words with Mr Gross, also said in its filing that he was warned he would lose US$200 million (S$271.3 million) in bonus if he quit before the end of the quarter.

Mr Gross departed Pimco in 2014 after a public falling out with other top executives as lagging results at the Pimco Total Return Fund, at the time the world's largest mutual fund, led to a flood of redemptions.

Mr Gross, who was Pimco's co-founder, sued a year later, claiming he was ousted so that Pimco wouldn't have to pay him his roughly US$200 million cut of the bonus pool and so other managing directors could increase their compensation.

Pimco is challenging the central claim in Mr Gross' lawsuit, which is seeking hundreds of millions of dollars.

"He had no employment agreement, and he acknowledged that leaving Pimco before the end of the third quarter of 2014 would render him ineligible for the profit-sharing payments he now claims he is owed," Pimco said in its first response to Mr Gross's allegations.

Pimco said Mr Gross met chief executive officer Douglas Hodge and general counsel David Flattum on Sept 25, 2014, the day before his exit, and was informed that he wouldn't get the bonus if he quit.

He left a handwritten resignation note the next morning, according to the filing.

"Very defensive, nothing remotely new," Mr Gross's attorney, Ms Patricia Glaser, said in response to the filing. "We're anxious to have the case proceed."

After Mr Gross' departure, Pimco's assets under management fell more than 20 per cent to US$1.43 trillion at the end of 2015. The flagship Total Return Fund has shrunk to US$87.8 billion from a 2013 peak of US$293 billion.

Mr Gross left behind a handwritten message "in the middle of the night, before walking out of Pimco's offices for the last time", saying his resignation was effective Sept 26, 2014, at 6.29am Pacific time, according to Monday's filing.

"Mr Gross ended his career at Pimco with no notice or transition, disregarding the potential impact on the individual and institutional clients whose assets he was responsible for managing," the firm said.

"He exited Pimco's Newport Beach offices for the last time without telling anyone he was leaving. His longtime colleagues first learned of Mr. Gross's departure from a press release issued by his new employer, Janus Capital Group, early on the morning of September 26, 2014."

In addition to Gross losing his bonus because of the timing of his resignation, Pimco said he has no grounds to sue because the firm "had good cause to terminate Mr. Gross's employment based on his conduct", according to the filing.

Pimco accused Gross of "trying to sabotage the careers of the former CEO and others he suspected of disloyalty, treating his colleagues abusively, and repeatedly threatening to resign without notice if he did not get his way".

Mr Gross, 71, whose estimated net worth is almost US$2 billion, has said he will donate any award or settlement from the lawsuit to charity.