A rent increase may be unlawful if it does not comply with the rules applying to your tenancy, including the permitted timing, calculation or notice requirements. Ask your landlord in writing to explain the increase and keep your tenancy agreement, rent records, notices and relevant correspondence. If it is not resolved, you can apply to the Residential Tenancies Board (RTB) for dispute resolution; any outcome will depend on the facts and evidence in your case. Do not withhold rent without getting advice, and consider speaking to a solicitor or Threshold about your position.