A study by Debenhams found that women keep their bras for, on average, five years, with nearly a third of those surveyed saying they've never been for a proper bra fitting.

Given that our busts change over time – for hormonal reasons as well as weight gain or loss – chances are your bra size has fluctuated too.

"Millions of women are wearing the wrong size bra and as market leader it’s our mission to help them find one that fits perfectly, whether it’s a first bra, a post-surgery bra or a pregnancy bra," says Julia Mercer, head of bra fit at Marks & Spencer.

But how do you know if you’re wearing the wrong size? Your bra might feel comfortable, but Mercer says there are five ways to tell, by looking at how it sits and fits on your body.

Follow these pointers to find out whether you need to think about swapping to another size.

1. Under band 
"The under band is most important area to get right, it’s the foundation of best bra fit," Mercer says. "You should only be able to pull the bra 4 or 5cms away from the back."

If your bra stretches further the band size may be too big or the bra might have lost elasticity, in which case it’s time to invest in a new one.

2. Front wires
"Wires at centre front should always sit back completely," Mercer says, so if there’s a gap, you’re wearing the wrong size.

3. Side wires
"Wires at side should sit on the rib cage and not breast tissue," Mercer advises.

If your bras rides up it could be due to either the band or the cup size, so you need to make sure both are right for you.

4. Cups
Suffering from the dreading 'double boob’ effect, where your bust is squeezed upwards? Chances are your bra cups aren’t big enough.

"Cups should fully encase the breast without spillage," says Mercer.

5. Straps
"You should always adjust the straps with your bra on," Mercer says, "and you should only be able to fit two fingers under the strap and your shoulder."

That’s why having a professional bra fit can help, because the assistant can adjust the straps to the perfect length for you.