Dame Joan Collins believes the ''key'' to a ''happy marriage'' is ''supporting each other'' through the ''highs and lows''.

The 86-year-old actress has been married to her fifth husband Percy Gibson for 17 years and she insists both people in a marriage must be there for one another through the best and worst of times and be able to communicate their emotions to one another.

She said: ''The key to a happy marriage is supporting each other. There will be highs and lows in any relationships, but make sure you are there for one another when you need it most.''

On the importance of communication in a relationship, she said: ''Communicate, communicate, communicate. I can't say it enough. You may not always agree on everything, but always make time to talk to each other every day.''

Joan also says there must be more to a romance than just lust as when the passion fades there must be more between a couple in order for them to stay together.

She said: ''Fall in love, not lust. People can fall head of heels for one another, then wake up one day and wonder why they fell for them in the first place. There has to be more, to be something to fall back on.''

The 'Dynasty' star also advises people to never ''settle for anything less than the best'' and to ''ditch and switch'' if you aren't being treated how you deserve to be.

She said: ''Don't settle for anything less than the best. If they don't treat you like you deserve, or you're not getting what you want, then it's time to ditch and switch, darling.''

Joan has teamed up with phone network Three and is calling on mobile customers to ''ditch and switch'' their current provider if they're not happy.

The playful social media campaign encourages the public to switch their phone provider by simply sending a free text message which allows them to move networks as quickly as one working day.

Joan stars in an advert for the campaign which sees her drawing on her dating experience to encourage viewers to take control and not stay in an unsatisfactory relationship with their mobile network.

The 'American Horror Story' star - who is famed for never settling for second best - is seen living the high life on-board a yacht in sunny St. Tropez, with playful nods towards the benefits of choosing 'the one', before she turns to the camera to say: ''I've never settled for anyone or anything which didn't float my boat ... It's now easier than ever to switch. So darling if they're not satisfying you, then just ... ghost them. So, take control - ditch and switch to Three.''

For more information about switching to Three, visit www.three.co.uk/switching