Parts of the Westminster parliamentary estate in London were on lockdown on Thursday afternoon following reports of a “chemical incident.”

Officers shut down areas to discover the source, and were investigating a “white powder.”

The substance is believed to have been found in an envelope addressed to a Peer, a member of the House of Lords.

A spokesperson for the Metropolitan Police says: "A suspicious package has been delivered to the Houses of Parliament.

"Officers are at the scene examining the contents of the package."

Parliament in lockdown. Letter to a peer being analysed by police over white powder in envelope. — Matt Chorley (@MattChorley) July 7, 2016

Doors and windows were closed and people, including some Lords, were reportedly locked out on the terrace.

The police say nobody was injured or made unwell in the incident.

"Chemical incident" on the Parliamentary estate. Police shutting down areas to discover source. Lords locked out on terrace — Tom McTague (@TomMcTague) July 7, 2016

Times reporter Matt Chorley was among those locked out, and was tweeting from the terrace.

@MattChorley in the toilets again? Isn't this becoming a little too regular? — Dan Lewis (@madeinhaste) July 7, 2016

Water now passed out on the terrace for those locked out by chemical incident. When should we be worried? "When you start turning green" — Matt Chorley (@MattChorley) July 7, 2016

Emergency supplies have arrived for the stranded on the Lords terrace #chemicalincidentpic.twitter.com/fAKonSjZEL — Matt Chorley (@MattChorley) July 7, 2016