The tiny primates have a distinctive high-pitched call

The thieves cut the power and broke the locks on the special enclosure at Jane Shields' home in the Eastbourne area of Darlington early on Friday morning.

The family is concerned that the mother and one baby in particular could die if they are not reunited.

The animals are worth about �1,000 each and Mrs Shields believes they may have been taken out of the area.

The monkeys are pets to Mrs Shields, her husband, and her 12-year-old son Alex.

These animals mate for life and family groups are essential for their collective wellbeing Jane Shields

Emma, who is Woody's mate and mother of the babies, was forced into an outside aviary during the theft and left behind because she would have attacked and been too difficult to handle.

Mrs Shields said: "They knew what they were doing. They obviously know how to handle monkeys, particularly marmosets which move very quickly.

"These animals mate for life and family groups are essential for their collective wellbeing.

"I am really worried that Fudge may not survive the ordeal of being separated from mum and the mother herself who is pregnant may abort because of the distress this incident has caused."

A Durham Police spokesman said the family was offering a substantial reward for their safe return, but they fear the monkeys may have been transported to the south of the country where they are more common.