As soon as I engage the blades the engine stops this has only just happened and it has been working fine until now you can start and drive it butnot cut This is a countax c600h thanks

I find no such control as "engage the blades" for this model. There is "blade control" which is a blade brake release and engine run. The blade is in motion any time the engine is in motion, including during start. You hold the blade control lever to start and operate the mower, then release lever to stop engine and blade. Follow all safety precautions.
https://www.mtdproducts.com/equipment/OperatorManualDisclaimerView?storeId=10500&catalogId=20500&langId=-1&pdfLink=http%3a%2f%2fmanuals.mtdproducts.com%2fmanuals%2f769-04527.pdf&htmlLink=

is this a honda ? what make model year ?

If it starts ok, the battery should be ok. The battery doesn't drive the mower, the gasoline engine that the starter works to start.
Do you have an owner's manual? see the troubleshooting section.
no manual try http://www.manualslib.com/
My guess is not enough fuel flowing to the engine due to a bad
governor (air blade from the cooling blower, or it's connections maladjusted. Otherwise, engague the clutch slowly. try not to engage blades at the same time.

Sounds like you have broken a shear pin in the drive pulley.
Shear pins are made to break if you hit something. Other wise it would cost a lot of money to repair the damage.

Check that the battery has good charge. If it is top up type check water level is above tops of plates. Run on mains powered external charger overnight. Fit red clip on positive + and black on negative - and ensure that you do not touch any metal frame parts with clips. If this happens there could be severe damage or a fire. The electric clutch requires a lot of current to stay engaged and can starve ignition coil for power if battery is too low. Some chargers cannot bring up a very low battery and drive an electric clutch.

Do the basics, check the battery voltage first. If there is not enough battery voltage present when you try to engage the blades it will certainly die and may not start until the battery is either charged or replaced. There are other items to check but do this first.

The transmission drive belt is a separate one from the blade belt. The clutch pedal when released puts tension on the transmission drive belt so it tightens up and drives the gearbox pulley by way of a large spring. Make sure the spring is ok. The belts don't last forever and probably needs replacing looking at the age of the machine. That is a fun job as it is right up under the machine and runs from the engine pulley right back to the top of the transmission. You have to drop the deck right off and I usually find a tree with a large branch and haul the front up with a rope or winch so the mower is vertical. Makes it easier to work on. Drain petrol first though. Hope this helps otherwise get a mower shop to do it for you if you feel it's to much of a task. Good luck.

sounds like a faulty safety switch

sounds like the safety switch is out of adjustment

This actually happened to a friend of mine! I saw it! Mow about 20 feet, and had to stop, then 20 feet...well, you get the picture. It was his air filter! Stopped up pretty good. We blew it out and it worked fine.
I really hope this helps. --W/D--