BIG Brother's pretend love birds got engaged today and were told they will be married on Sunday.

The proposal by Mario Marconi, 42, to Stephanie McMichael, 19, was all part of a secret challenge set by Big Brother, when the couple entered the house. The challenge is to convince other housemates they are a couple.

Having never met each other before that evening, the task was made all the more complicated because Mario's real-life partner, Lisa Appleton, 40, is a fellow housemate.

The only other contestant in on the challenge is politics student Luke Marsden, 20.

If the four succeed in fooling their fellow contestants they will be guaranteed immunity from the first eviction.

This afternoon Mario summoned fellow contestants to the swimming pool where he dived into the water to emerge holding a ring and proposing to Stephanie.

After taking her time to reply, Mario joked: “Why not? It's a better place than nowhere.” Stephanie agreed.

In one final twist, Big Brother told housemates that the wedding was going to take place in the house on Sunday.

Stunned

Looking visibly shocked at the news, Stephanie said: “Oh my God!”.

However, earlier in the day the deceit was proving difficult to maintain and both Mario and Stephanie retired to a bedroom together for some “us time”.

When alone Stephanie asked: “What if they corner us and say 'give him a kiss' at some point? I can see it coming.”

Mario agreed that it: “Would be the killer.”

The pair were nearly rumbled earlier in the day by Rebecca Shiner, 21, who asked if the faux-couple were going to have sex in the house.

The prompt denial delivered simultaneously by the pair led Rebecca to ask: “It's not a task is it? I thought you were lying and it was a task or something you've been told to do.”

Mario quickly changed the subject and Rebecca let the matter rest.

Alone

Lisa spoke last night about how she is finding life without her real-life partner Mario.

Speaking in the Diary Room, she said: “I'm dying to grab him or give him a big hug but I can't.”

On the subject of the sleeping arrangements she was more pragmatic and acknowledged that other housemates might suss if her boyfriend did not sleep in the same bed as his pretend girlfriend.

Guilty

However, she said Stephanie does not need to feel guilty because: “It's just a task, and it's a good one. I wasn't expecting that.”

But Stephanie was not so keen to share a double bed with Mario - who is more than twice her age – and admitted that a few of the housemates are “sceptical” about her pretend relationship.

One housemate convinced by the ruse was Rachel Rice, who told Big Brother: “I love Mario and Steph, she's beautiful. The boys will be wanting a bit of Steph but they're not going to get her - Mario is really nice.”

But the charade is putting a dampener on Stephanie's other potential romances inside the house.

Darnell Swallow, chatting to Mohamed Mohamed in the garden about the most attractive girls in the house, said: “Steph would be up there as well but she's got a man and I'm not a home-wrecker.”