They said the secrecy of the vote was compromised in some polling stations because of overcrowding or people simply failing to obey the rules.
Observers watched last week’s general election taking place in eight constituencies, including Birmingham Ladywood.
In some cases, more than one person went into polling booths - making it next to impossible for votes to be secret.
Sebastian Pillay, an MP from the Seychelles and head of the observer team, said: “The findings show there is a strong democratic system, yet there is room for improvement in the UK electoral process to ensure the trust that exists in the system is continuously reinforced.”
The observers, known as an Election Assessment Mission, included 32 politicians and officials from Commonwealth countries including Anguilla, Antigua and Barbuda, Australia, Bangladesh, Bermuda, British Virgin Islands, Canada, Ghana, Guyana, India, Jamaica, Kenya, Montserrat, New Zealand, Pakistan, Seychelles, Sierra Leone, South Africa, St. Lucia, Tonga, Uganda and Zambia.
And in an early report, they said: “The often inconsistent application of polling station procedures and guidance throughout different constituencies and within the same constituency was highlighted by the Observers.
“This was deemed contrary to the certainty and consistency of the voters’ experience on Election Day and in some cases to the consistency and integrity of the secrecy of the vote.”
They added: “In nearly 10% of observed polling stations, the secrecy of the vote was considered to be compromised, due in some cases to: overcrowding, polling station and polling booth layout, voter carelessness regarding the folding of the ballot, and insufficient control by polling station staff regarding the behaviour of voters in the booths, including several direct observations of more than one person in the booth.
“In 20% of observed polling station the procedures regarding the identification of electors, namely the reading out of their name and voter number were not carried out in the period of observation.
“The sometimes inconsistent application of procedures was largely attributed by the Observers to unclear instructions.”
Overall the election was conducted well, the observers said.
The initial report said: “Despite the nature of the snap election and the short planning window for election administrators, the poll was well administered according to the Observers’ findings.”
And they said: “The Observers noted that the campaign was particularly contested despite the short time available to plan and finance campaigns, and that citizen engagement was very high.”
A more detailed report will be published in the summer.
Observers looked at the conduct of the general election in eight constituencies across the country including Birmingham Ladywood, Darlington, East Devon, Edinburgh West, Gower, Hendon, Manchester Central and North Norfolk.
They watched voting in 267 polling centres, and saw votes counted at eight centres.
The Commonwealth sends missions made up of MPs and election officials to examine the administration of elections in member states across the world.