An engineer accused of assaulting a lawyer outside one of Parliament's bars told a court the complainant called him an "imbecile" and an "idiot" in an exchange after they had both been drinking.

Ronald Freeman, 57, of Borehamwood, said Peter Brooksbank had sworn at him in a "posh derogatory" manner in a smoking area outside the Sports and Social Bar within the grounds of the House of Commons in London on December 5 last year.

Freeman, who denies one count of assault occasioning actual bodily harm, said he had approached a "dishevelled" Mr Brooksbank as the lawyer sat on a step, putting his hand on his shoulder to check if he was alright.

But Freeman, giving evidence at Southwark Crown Court, said Mr Brooksbank had reacted aggressively.

He said: "He started shouting at me, 'What do you think you're doing?'"

He said despite another contractor stepping in to "calm" the situation, Mr Brooksbank was "still swearing, in a sort of posh way, posh derogatory".

He added: "There was the f-word, f***ing idiot, imbecile.

"First he was shouting and then it got to a point where he was sort of snidely saying things."

Freeman said he had been attending a reunion dinner in the Strangers venue at Parliament with current and former engineers and had drunk around four glasses of red wine and a pint of Guinness before the first interaction with Mr Brooksbank.

Mr Brooksbank was seen on CCTV at the Sports and Social Bar at 3.40pm.

The trial has heard he had consumed around three pints of Guinness before the incident.

Mr Brooksbank, a lawyer who works as deputy counsel to the joint committee on statutory instruments, a group of MPs and peers that scrutinises secondary domestic legislation, needed between 12 and 20 stitches to his head after the alleged assault.

He has told how, following their first encounter in the smoking area, Freeman, who had worked at Parliament for 11 years, later yanked the Parliamentary pass off his neck before taking it to another part of the building.

Mr Brooksbank followed the defendant and retrieved his pass before exiting the room, and was then allegedly assaulted in an alleyway, the court has heard.

The complainant said he was shoved from behind, causing him to hit his head on paving stones and was left swollen and "black all over".

On Wednesday, Labour MP Andy Slaughter took to the witness box to tell the court he felt compelled to give evidence about an incident around a decade ago when he said he was shoved on an escalator and shouted at by Mr Brooksbank.

Mr Slaughter, the representative for Hammersmith, said he first knew about this case having read a story in the Metro newspaper this week.

The MP, who said he does not know and had never met Freeman, said he thought he had information which may be "pertinent" to the trial.

He told the jury he had been standing on the left side on an escalator at Westminster Tube station, in conversation with someone else, on his way to Parliament when he encountered Mr Brooksbank.

He said: "While we were moving upwards on the escalator, we were stationary, I suddenly received a hard push, a hard shove and then someone shouted at me which was quite a shock."

While he could not recall the exact words shouted at him, he said the tone was aggressive.

He recognised Mr Brooksbank from a select committee he had previously sat on and said he was shocked at the "inappropriate behaviour".

He said: "I was shocked that it was someone who was an officer of the house, somebody who had an important position there and I just thought it was totally inappropriate behaviour."

The court heard Mr Brooksbank on that occasion accepted he called the MP a "f***ing idiot".

The lawyer wrote a letter of apology to the MP following an investigation by the Serjeant at Arms.

Mr Slaughter, who said he did not keep the letter, said it seemed to be "a very begrudging and sarcastic apology".

The trial continues.

Read more: Lawyer 'left black all over after being knocked out at House of Commons'