The long civil conflict has left a trail of dead, missing and displaced in its wake
MPs from all parties are calling for an end to all UK military aid to Colombia, citing murders and human rights abuses by the country's security forces.
The MPs have launched a TUC-backed group called Friends of Colombia.
Thousands of trade unionists and human rights activists have disappeared or been killed or jailed in Colombia.
The UK Foreign Office said it shared many of the MPs' concerns, but added: "We do not believe that isolating Colombia will help solve its problems."
"Quite the opposite, we believe that we have a strong interest in helping Colombia address its problems of violence and exclusion, human rights abuses and illegal drugs. We cannot turn our back on these problems," the Foreign Office said.
Campaigners and MPs say they do not want to isolate Colombia, but to divert military aid to humanitarian work.
Millions of Colombians have been displaced by the civil conflict there, which spans more than four decades and involves left-wing rebels, right-wing paramilitaries and drug traffickers.
Some 109 trade unionists have been killed since 2007, with teaching being the worst affected sector.
This month two members of teaching union Asedar were killed in the same week in the northern Arauca region. The first, Pablo Rodriguez Garavito, was shot by gunmen in a classroom where he worked.
Other groups such as students, journalists, agricultural and oil workers are also regularly targeted, threatened and killed in Colombia.
The Colombian authorities say the death toll is lower than humanitarian and foreign governments report, and it is working to improve human rights and levels of impunity in the country's military.
In Colombia an inquiry is currently under way into the cases of some 1,500 civilians believed murdered by troops since 2004.
In May 20 soldiers were arrested and accused of killing civilians and presenting them as rebels or paramilitaries to inflate army statistics in their security drive.
EDM - COLOMBIA MILITARY AID
notes with concern the increase in trades unionists assassinated in Colombia in 2008
notes the recent statement by the UN High Commissioner for Human Rights that she believes the Colombian security forces to be involved in "widespread and systematic" killings of civilians
welcomes the findings of the Foreign Affairs Select Committee... [in particular the] conclusion that UK military assistance to Colombia is "inappropriate" given the "serious" human rights situation in Colombia
calls on the government to freeze UK military assistance to Colombia until the regime fully implements the repeated human rights recommendations made by the UN
Last year, 27 soldiers and officers were fired when the scandal came to light.
The launch of the new parliamentary group of MPs on Tuesday heard from director of human rights at Colombia's TUC-equivalent, the CUT (Central Unitaria de Trabajadores), Luis Alberto Vanegas.
He said despite claims to the contrary the human rights situation in Colombia was "getting worse". As well as assassinations the CUT had registered more than 550 different types of human rights violations, he said.
He said five members of the CUT's executive committee had received death threats by email, phone and by delivery of mock funeral wreaths to their offices.
"I myself have been harassed and increasingly followed by cars. I worry that this could end in assassination without forewarning," he told the BBC.
General secretary of the British Trades Union Congress (TUC) Brendan Barber told MPs at the meeting that the "appalling" levels of human rights abuses would be further highlighted by the new group.
"This represents a stepping up of Parliamentary activity," he said.
The group agreed to request a meeting with Foreign Secretary David Miliband, lobby MEPs about trade with Colombia, and co-ordinate questions to the Foreign Office about how much it was spending on military aid and whom was benefiting.
The UK has never revealed the total amount it has spent on anti-drugs work with the Colombia army, citing "security concerns".
A Parliamentary Early Day Motion (EDM) on aid to Colombia has attracted 242 MPs' signatures, making it the fourth most supported of 1,891 EDMs.
The motion - sponsored by Manchester Central MP and chair of the new group Tony Lloyd - refers to a statement by the UN High Commissioner for Human Rights, who believes the Colombian security forces to be involved in "widespread and systematic" killings of civilians.
It also welcomed the findings of the Foreign Affairs Select Committee which said UK military assistance to Colombia was "inappropriate" given the "serious" human rights situation there.
In March Foreign Secretary David Miliband announced an end to bilateral aid projects with the Colombian Ministry of Defence, which it said had been aimed at "human rights and humanitarian de-mining" projects with the country's army.
But he said the UK would continue to fund other human rights work and counter-narcotics projects, including a £900,000 contribution to a UN programme.
Mr Vanegas came to Britain to urge the UK to help stop human rights violations
In his statement Mr Miliband said the government "shared the concerns of many in the House that there are officers and soldiers of the Colombian Armed Forces who have been involved in, or allowed, abuses".
But UK-based humanitarian group Justice for Colombia (JFC) said while it welcomed the partial change in policy the UK's continued involvement in counter-narcotics projects remained and lacked transparency.
A JFC study on military aid said the Colombian military made "no distinction between counter-narcotics and counter-insurgency operations", and used both as a pretext for targeting civilians, trade unionists and human rights activists.
General Secretary of the UK's National Union of Journalists, and chairman of JFC, Jeremy Dear said it was "unacceptable" for any foreign country to provide aid to the military.
"Colombia does need foreign assistance, but for things such as employment, housing, health and education projects - things that will really benefit the people," he said.