CEO Ralph Findlay admitted planning for Brexit had been a "distraction" and said it doesn't want any "barriers" in the way as it looks to grow its export business.
It comes as the Wolverhampton-based group, which has pubs across the West Midlands, reported a pre-tax loss of £20 million for the 12 months to September 28 2019 after making a profit of £54.3 million in the prior year.
However, its revenue rose from £1.14 billion to £1.17 billion over the same period.
Mr Findlay said: "The political uncertainty doesn't help and planning for Brexit has been a distraction.
"It is important it is resolved quickly and in the right way, which will be a positive for UK businesses.
"We do import significant levels of food and drink, and we are looking to grow our export business so we don't want barriers in the way."
In its financial update, Marston’s noted the impairment of underperforming properties during the year, which resulted in a £43.4 million charge.
But it said it was ahead of schedule in its plan to cut £200 million from its debt pile in 2020-2023.
Mr Findlay said: "Back in January we launched our review strategy, and with the political and economic uncertainty in mind, we prioritised debt reduction and cash flows.
"We are making good progress with our debt reduction plans and are ahead of schedule in meeting the accelerated £70 million of disposal proceeds which we are targeting in the current year.
“We continue to benefit from Marston’s balanced business model and our Taverns wet-led community pubs and brewing businesses have both once again outperformed the market, building on an outstanding year last year. We are employing a renewed focus on the proposition in our food-led pubs and remain well placed to benefit from reduced supply in this market segment, of which there is beginning to be some evidence.
“Our principal focus remains to reduce our net debt by £200 million by 2023 – or earlier – and the measures we are taking now will result in a high quality business which is cash generative after dividends and capital expenditure.
"It is unlikely we will be buying new pubs for the time being. The focus is to get more from the assets that we own. We are spending £85 million this year on our pub estate, making them modern and improving their offer."
The group said trading was "on track" for the initial weeks of the current year and it is "well prepared" for the festive season.
During the year the company opened eight new-build pub-restaurants, 15 wet-led pubs and two lodges.
It also completed the sale of 137 pubs to Admiral Taverns for £44.9 million.