Strictly speaking no.  A tenancy is a contract and under contract law, once a contract has been agreed, it can only be changed by consent.  You have confirmed your agreement by paying the security deposit.

However as you have not signed the tenancy agreement yet, you are in a weak position and may in practice find it hard to hold the landlord (through his agents) to his agreement.  If you think you can find somewhere else to live this may be better – as do you really want to rent from someone who behaves like this?  

However, if you are anxious to rent this particular property you may find it difficult as the procedures available to you to challenge the landlord’s action – making a complaint to the agents’ Property Redress Scheme or seeking a court order, will all take a lot of time and are not a practical option.  

Probably your best option would be to sign the tenancy agreement (as otherwise, you will not be able to rent the property) and then make a complaint to the agent’s Property Ombudsman Scheme (see our article below). 

However, only do this if you can afford the higher rent and be prepared for them to find that as you have signed the tenancy agreement you have consented to the rent increase and that no redress is available to you.