Generally speaking, things published in a 'quality' national newspaper like The Guardian can usually be regarded as reliable as they are written by professionally-regulated journalists and newspaper articles are also copy-edited, which might sometimes involve fact-checking . Journalists don't have to state all their sources like academics do (but they do need to be able to prove them later, if anyone questions the validity of their work).
If you are using a newsaper article as a short cut to factual information that it otherwise freely available in peer-reviewed academic texts in your field of study, then the person marking your work might see this as corner-cutting and mark you down appropriately. So your best bet is to run a Google Scholar search to see if you can track down an alternative source.