fatal error Cannot redeclare get_temp_dir()

Hi,

I went to update wordpress today on a clients website before doing some SEO work for them and now when ever I go to login to the dashboard I get the following error.

Fatal error: Cannot redeclare get_temp_dir() (previously declared in /var/www/vhosts/aird-cottage-skye.co.uk/httpdocs/wp-includes/functions.php:2160) in /var/www/vhosts/aird-cottage-skye.co.uk/httpdocs/wp-admin/includes/file.php on line 189

The website itself is running fine but I just can not access wordpress.

I really need this fixing asap as they have paid for their SEO and I need to login today to complete.

Any help would be much appreciated.

Many Thanks