LONDON—Detectives are probing "very serious" allegations that a police officer fabricated claims against a U.K. lawmaker who was forced to quit a Cabinet post after being accused of insulting police with the term "pleb," the prime minister said Wednesday.

Andrew Mitchell quit as the government's chief whip after he was accused of hurling the word "pleb" at officers who stopped him from riding his bicycle through the Downing Street gates in September. Mitchell admitted swearing, but denied calling anyone a "pleb"—a demeaning reference to working-class people that touched a nerve in this class-conscious country.

Last week, a police officer from the diplomatic protection squad was arrested for alleged misconduct in public office. Channel 4 News has claimed the officer emailed his local lawmaker falsely pretending to be a member of the public who had witnessed the altercation. The email claimed Mitchell had sworn repeatedly and called officers "plebs," shocking passers-by.

"A police officer posing as a member of the public and sending an email potentially to blacken the name of a Cabinet minister is a very serious issue and does need to be seriously investigated," British Prime Minister David Cameron told lawmakers in the House of Commons on Wednesday.

Channel 4 News also showed CCTV footage Tuesday that appeared to contradict the officer's claim that there were other witnesses to the spat.

The Metropolitan Police said 30 officers had been assigned to investigate the incident. "It goes to the very heart of the public's trust in the police service," the force said in a statement.

The statement said both the force and the police watchdog were investigating the arrested officer's conduct and whether there was any evidence of a conspiracy between the arrested officer and anyone else.

Allies of Mitchell said there is mounting evidence he had been falsely accused, and called for him to return to the Cabinet. He resigned as chief whip a month after the Downing St. altercation but remains a member of Parliament.

The lawmaker admitted swearing, but strongly denied using the word "pleb," which reinforced a perception of the governing Conservative Party as elitist and lacking empathy with the poor.

"I would never call someone a (expletive) pleb," Mitchell told Channel 4 news on Tuesday. "Anyone who knows me well would know that it is absolutely not in me to use phrases like that."