Jennifer Aniston and Justin Theroux will reportedly marry this weekend.

The 'We're the Millers' actress and her fiance are so determined to keep their nuptials secret, friends and family invited to their Los Angeles home on Saturday (10.08.13) have been told it is for Justin's 42nd birthday party.

The couple - who met on the set of 2011 movie 'Wanderlust' - will then celebrate their marriage with a bigger party later in the year.

A source told the Daily Mirror newspaper: ''August 10th is Justin's birthday, and their nearest and dearest have been invited to a huge 'birthday party'. But everyone is convinced they're planning to exchange vows within the grounds of their new home and for it to be a huge surprise on the day.

''They have gone to extreme lengths to keep it quiet - openly lying to their friends to keep up the ruse. Apparently it's going to be a very low-key affair with the possibility of a bigger celebration later on in the year.''

It is believed the 44-year-old actress - who was previously married to Brad Pitt - and Justin told their mothers Nancy and Phyllis, about their plans over a family dinner in New York two weeks ago, and both women will be at the wedding, despite Jennifer's strained relationship with her mum.