If a cohabiting couple later marry or if you are already married, the Declaration of Trust will be superseded by the Matrimonial Causes Act 1973.
The Court has the power to determine the distribution of assets between the married couple and this could mean overturning the Declaration of Trust. However, the Court would consider any Declaration of Trust in place as evidence of the couple’s intentions.
In some cases, the best way to seek to protect your interests would be to enter into a pre and/or postnuptial agreement.
This will also apply to civil partnerships.