How do you answer ‘What is your current salary’?

For background, I’m interviewing for jobs that are 10-30k more than my current salary. I believe I am suitably qualified and the pay rise is justified.

However, on learning my current salary, employers tend to get hesitant or ask if I’d accept a lower salary. I argue my point about the market rate, my expertise, etc but I’m not sure if it convinces them.

How can I best answer a direct question like ‘What is your current salary?’ without giving the answer but also not sounding defensive?

Edit: The general consensus seems to be to lie about the salary. I’ve asked my HR department what information they share on references as I’d be worried about getting caught out at some point. I’m also terrible at lying but that’s for me to work on!