Conservative MP Johnny Mercer secretly travelled to Ukraine's capital Kyiv in recent days, he has revealed.
On Friday night, Mr Mercer, a British Army veteran and former Veterans Minister, said he boarded a plane to Krakow "a week ago" and headed back to the West on Wednesday.
The Plymouth Moor View MP tweeted photographs of himself visiting injured people in hospital, adding he witnessed "total carnage" alongside demonstrations of "incredible human spirit".
Mr Mercer told The Times: "I didn't tell anyone, I just disappeared. I decided it was the right thing to do."
Writing in the Daily Telegraph, he said he made the decision to travel after receiving an invitation from a former MP in the Donetsk region.
Government advice warns against travelling to the country.
Mr Mercer is the first UK MP to visit the country's capital since the outbreak of war.
Watch:Has Russian progress in Ukraine been slower than expected?
Conservative MP Adam Holloway was criticised by Downing Street last week for visiting a different part of Ukraine.
Mr Mercer said he also visited Lviv, where he said he met with local politicians.
The Ministry of Defence has banned UK military personnel from travelling to Ukraine, with the Defence Secretary stating any serving personnel who go to Ukraine "will be breaking the law and they will be prosecuted when they return for going absent without leave or deserting".