Police investigating a suspicious letter delivered to an office in the Houses of Parliament found it contained white powder which was "non-harmful", the House of Commons said.
Counter-terror detectives from the Metropolitan Police were called in to look at the package and found it was not dangerous.
There were few signs of concern around the Palace of Westminster as reports began to emerge of a heavy police presence in the ministerial corridor.
Relatively few MPs and peers will have been in work as both the Houses of Commons and Lords are in recess, although Parliament is also home to thousands of staff and journalists.
A Commons spokeswoman said: "Today the Metropolitan Police investigated a small package containing white powder on the Parliamentary Estate.
"The powder was found to be non-harmful."